JPH08195720A - Wavelength selection filter for pill box type optical resonator - Google Patents

Abstract

PURPOSE: To facilitate the design by arranging a pill box type optical resonator and input coupling wave guide path along outer periphery of the pill box type optical resonator at an equal interval by a prescribed angle so as to improve the control performance in coupling characteristic. CONSTITUTION: The pill box type optical resonator 1 is resonated at wavelengths nearly at an equal interval in the whisperring gallery mode being a natural mode. For example, when the wavelength of a light incident from a port P-1 of an area A is close to a resonance wavelength, the light is coupled with the resonator 1 and the energy is outputted from a port P-2. On the other hand, when the wavelength is apart from the resonance wavelength, the coupling of the signal with the resonator 1 is weak and the energy is outputted directly from a port P-3. Then the filter acts like a branching circuit extracting only the light of the resonance wavelength at the port P-2 selectively. The pass band width and the block band width of the filter are set based on a coupling degree decided by a distance between a guide path 2 or 3 and the resonator 1 and an angle coupled at an equal interval.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength selection filter for demultiplexing or combining light waves having a plurality of wavelengths used in an optical frequency multiplex transmission system, and improving the selection characteristics of light waves having a narrow wavelength interval. Another object of the present invention is to provide a demultiplexer or a multiplexer used in optical frequency multiplex transmission type optical communication in which the selection characteristics can be easily controlled.

[0002]

2. Description of the Related Art Conventionally, a pill box type (or disc type,
Several academic studies have been conducted on (disk type) optical resonators. However, since this resonator is essentially a super multimode resonator, little practical research has been done.

[Problems to be Solved by the Invention]

Regarding the optical filter using the pillbox type optical resonator, there are some research reports on the resonance characteristics of the pillbox type optical resonator alone, and the pillbox type optical resonator and the linear waveguide are coupled. There are reports on the structure. However, in the case of these resonators and coupling systems, the resonance wavelength is fixed, the resonance wavelength interval is narrow,
Furthermore, since the structure of the coupling portion between the optical resonator and the linear waveguide is not uniform over the coupling range, it is not easy to design the coupling characteristic and it is difficult to obtain a desired wavelength selection characteristic. Therefore, there is no example realized as a practical optical filter.

An object of the present invention is to realize a wavelength selective filter for demultiplexing or multiplexing light waves frequency-multiplexed at narrow wavelength intervals. Further, by adopting electrodes, controllability is improved and many applications are achieved. It is an object of the present invention to provide a pillbox type wavelength selective filter applicable to the.

[0005]

According to the present invention, a pillbox type optical resonator and an input / output coupling waveguide are arranged along a circumference of the pillbox type optical resonator at a predetermined angle at equal intervals. The pillbox type wavelength selective filter is characterized in that the resonance wavelength is continuously variable by providing an electrode on a part thereof.

In implementing the present invention, a plurality of the wavelength selection filters are connected in cascade.

The present invention solves the conventional problems by adopting a structure in which waveguides for input and output are arranged at equal intervals along the outer circumference of a pillbox type optical resonator. The adoption of this structure is the first report of the present invention as far as the inventors know, and the use of electrodes for controlling the resonance characteristics of the pillbox type optical resonator has not been performed at all. These new inventions made it possible for the first time to use it as a practical optical filter.

The present invention comprises a pillbox type optical resonator (I), input / output coupling waveguides (II) and (III), and an electrode (IV) provided in a part of the pillbox type optical resonator. Composed. Here, the pillbox type optical resonator is composed of a disk-shaped medium having a higher refractive index than the surrounding medium,
Light propagates along the inside of the outer periphery while repeating reflection inside the outer periphery of the whispering gallery mode (whisp
ering gallery mode).

The present invention has, as its basic constituent elements, one pillbox type optical resonator 1 and input / output coupling waveguides 2 and 3 arranged along it, as shown in FIG. This basic component is composed of a medium having an electro-optical effect, and
As shown in (A) and (B), an electrode 4 is partially provided along the periphery of the pillbox type optical resonator.

FIG. 2A shows the case where the electrode 4 is provided at the non-coupling portion of the resonator 1, and FIG. 2B shows the case where the electrode 4 is provided at the coupling portion of the resonator 1. The resonant wavelength is continuously variable by changing the refractive index of the medium by the voltage applied to the electrodes. Further, as shown in FIG. 3, by connecting a plurality of wavelength selection filters in cascade, a wavelength selection filter for individually selecting the optical wavelengths of multiple channels is realized.

The cross-sectional dimensions of the coupling waveguides arranged along the pillbox-type optical resonator and the distance between the coupling waveguides and the pillbox-type optical resonators connected in series are determined by the waveguide mode of the waveguide and the whisper of the resonator. The ring gallery mode is chosen to meet and combine the phase matching conditions.

The electrodes 4 are arranged at the non-coupling portion of the pillbox type optical resonator as shown in FIG. 2A or at the coupling portion as shown in FIG. 2B. Two
There are types. When it is arranged in the non-coupling portion, only the resonance characteristic can be controlled, and when it is arranged in the coupling portion, both the coupling characteristic and the resonance characteristic can be changed.

The operating principle of the wavelength selective filter of the present invention shown in FIG. 1 is as follows. The pillbox type optical resonator of FIG. 1 resonates at wavelengths at substantially equal intervals due to the above-mentioned whispering gallery mode, which is its eigenmode. Therefore, for example, the light incident from the port P-1 in the region A is coupled with the pillbox type optical resonator when the wavelength is close to the above resonance wavelength, and the energy is output from the port P-2. . On the other hand, when the wavelength is far from the resonance wavelength, the coupling with the resonator 1 is weak and the light is directly output from the port P-3. Therefore, this structure is a demultiplexing circuit that selectively extracts only the light of the resonance wavelength to the port P-2. The pass band width and the stop band width of this filter are set by the coupling degree determined by the distance between the waveguide 2 or 3 and the resonator 1 and the angle (coupling angle φ) of the portions connected at equal intervals. it can.

The resonance wavelength of the pillbox type optical resonator 1 is
It is determined by the refractive index of the medium forming the resonator 1 and the dimensions (diameter and thickness) of the resonator. Since the electrode 4 is provided in a part of the pillbox type optical resonator, the resonance wavelength is continuously variable by changing the refractive index with the voltage applied to the electrode 4.

[0015]

EXAMPLE An example of the characteristics of the optical wavelength selection filter theoretically calculated assuming a wavelength band of 0.9 μm is shown below. here,
Proton exchange LiNbO _{3 is used} as a medium forming the resonator.
, The pillbox radius is 200 μm, the distance between the pillbox resonator and the waveguide is 0.6 μm, the waveguide width is 0.8 μm,
Calculation was performed assuming a bond angle of 0.1491π.

FIGS. 4A and 4B show the calculation results of the output light intensity at the port P-2 and the port P-3 when the light is incident from the port P-1 in FIG. 1, respectively. The resonance wavelength interval Δλ is 0.2824 nm, which shows the wavelength selection characteristic in which the light output is at the port P-2 only at the time of resonance.

This optical wavelength selection filter has the configuration of FIG.
Calculation results of the wavelength selection characteristics when individually connected are shown in FIGS. In this example, the wavelength selection filter 11 to the wavelength selection filter 13 are of exactly the same design, but no voltage is applied to the wavelength selection filter 11, and the wavelength selection filter 12 and the wavelength selection filter 13 respectively, A control voltage is applied so that the resonance wavelength shifts by Δλ / 4 and Δλ / 2. Light of four wavelengths (λ / 4) with a wavelength interval of (5/4) Δλ from the incident port
It can be seen that when ₁ to λ ₄ ) is incident, light of each wavelength is separated and output to each port.

Since the resonance wavelength is controlled by applying a voltage,
The characteristics of the wavelength selection filter of the optical resonator can be easily controlled, and a wavelength selection filter suitable for practical use can be realized.

[Test Example] (1) Preface As a light filter that is one of the devices that form an optical integrated circuit, the present inventors have focused on a pillbox type optical resonator having a thin disk-shaped core. , And have conducted analytical and numerical studies on the characteristics of the resonator itself. The eigenmode of this resonator is the whispering gallery mode, which is a super multimode. Then, in consideration of actually constructing a filter, we proposed to use a coupled waveguide for the input and output of light waves, and calculated the gap with the resonator and the width of the waveguide to efficiently transfer the optical power. .

Therefore, in the present invention, the wavelength dependence characteristics of the passive element composed of the pillbox type optical resonator and the coupling waveguide are calculated. As a calculation method, the analysis target is divided into several regions, and the light input / output state in each region is represented by a scattering matrix. This scattering matrix is roughly divided into two types, mode conversion and mode propagation. The former calculation requires a mode field distribution, and the latter calculation requires a propagation constant. These are calculated by eigenvalue calculation based on FE-BPM. I asked. Furthermore, as an application, we calculated the wavelength-dependent characteristics of the structure in which multiple passive elements are combined, and showed that it can operate as a wavelength selection filter.

(2) Analytical Structure In FIG. 1 (A), the element is formed in the plane of the substrate and originally has a three-dimensional structure. However, in the present invention, the element is arranged in the thickness direction (z direction) of the element. Assumes a TM single mode confinement and applies an equivalent refractive index method for two-dimensional analysis. Therefore, in the waveguide, z
In the TE single mode of the slab waveguide, which is uniform in the direction, and in the pillbox type optical resonator, the electric field component is the WGH mode having only the component perpendicular to the substrate. The refractive index of the core and the waveguide and the clad of the pillbox type optical resonator are N _CO , N _ow , and N _Cl , respectively, and the pillbox radius is a
And

Then, it is roughly divided into four regions depending on the positional relationship between the pillbox type optical resonator and the waveguide. In regions A and C, the pillbox type optical resonator and the linear waveguide are sufficiently separated from each other, so that the coupling can be ignored. In areas B and D,
The pillbox type optical resonator and the curved waveguide are kept at a constant distance and are coupled by a coupling angle φ ₁ .

Now, the operation of this element will be considered. First, input light is given from the port P-1 to the linear waveguide. Then, when propagating from the area A to the area B, the optical power is converted into an even mode and an odd mode which are eigenmodes in the coupling portion. Strictly speaking, since the pillbox type optical resonator is multimode in the radial direction, there is also a coupling between this higher-order mode and the mode of the bending waveguide, but its effect is small, Consider only coupling with modes. Therefore, in the region B, two mutually orthogonal modes propagate according to their propagation constants, and the phase changes. The mode is converted again at the boundary between the regions B and C, and propagates as the eigenmode of the pillbox optical resonator and the eigenmode of the linear waveguide. Then, in the region C, the eigenmode of the linear waveguide is output to the port P-3, and the light wave propagates along the outer circumference of the core as a whispering gallery mode which is an eigenmode in the pillbox type optical resonator. Then, again in the region D, the eigenmode of the coupling portion is converted and propagates. Then, when reaching the boundary with the region A, mode conversion is performed again, the eigenmode of the linear waveguide is output to the port P-2, and the eigenmode of the pillbox optical resonator reaches the region B again after propagation. In this way, by coupling the two waveguides with the pillbox type optical resonator interposed therebetween, the resonance characteristic of the pillbox type optical resonator can be utilized.

(3) Analysis Results The input / output characteristics of the coupled system shown in FIG. 1 (A) are calculated. The parameters used in the calculation are the pillbox radius a = 200 μ
m, and the proton effect LiNbO ₃ , assuming that the core of the pillbox type optical resonator and the refractive index of the waveguide and the cladding are N _CO = N _ow = 2.302 and N _Cl = 2.201, respectively, and the gap between the core and the waveguide. G = 0.6 μm, waveguide width W = 0.
8 μm, bond angle φ ₁ , power transfer rate at the joint is F = 0.2
Was set to 0.1491π. In the present invention, 0.9 μ
It is assumed that the m-band light is treated, and the circumferential mode order of the pillbox having the resonance wavelength closest to 0.9 μm is 3190.
Is.

The results of the numerical calculation are shown in FIGS. 4 (A) and 4 (B).
Shown in In both figures, the horizontal axis is the wavelength of the input light given to the port P-1, and the vertical axis is the normalized output powers | S ₂₁ | ² and | S ₃₁ | ² to the ports P-2 and P-3, respectively.
The output power is standardized with the input power.
FIGS. 4A and 4B show that when the input wavelength does not match the resonance wavelength, the light is output to the port P-3, but is output to the port P-2 only at the time of resonance. . Further, the resonance wavelength interval Δλ = 0.2824 nm can be obtained from the figure, which is a more reasonable value than the circumferential mode interval Δλ≈λ / (h _b −1) = 0.2823 nm of the pillbox type optical resonator.

Next, it will be considered to combine a plurality of coupling systems having such characteristics to operate as an optical filter having a structure as shown in FIG.

A scattering matrix of FIG. 3 is shown in FIG. First, all pillbox radii used should be the same. As it is, it has the same resonance characteristics but does not operate as a filter. Therefore, by attaching an electrode to each pillbox type optical resonator and applying a voltage, the electro-optic effect is used to change the refractive index only in that part. As a result, the optical path length is changed to shift the resonance characteristics.

A schematic diagram thereof is shown in FIG. In this case, it is assumed that there are four input wavelengths to be filtered. First, the pillbox type optical resonator 11 is set to a reference state in which no voltage is applied. Next, the pillbox type optical resonator 12 adjusts the applied voltage so as to have a resonance characteristic that is shifted by ¼ of the resonance wavelength interval Δλ. The pillbox type optical resonator 13 is Δλ /
The applied voltage is adjusted so that the resonance characteristic is shifted by 2. In this way, when the resonance states of the three pillbox optical resonators are shifted, the input wavelengths λ ₁ , λ ₂ , λ ₃ , λ ₄
As shown in FIG. 7, when λ ₁ is used as a reference and the wavelength interval is 5Δλ / 4, λ ₁ , λ ₂ , and λ ₃ are equal to the resonant wavelengths of the pillbox-type optical resonators 11, 12, and 13, respectively. I am doing
λ ₄ does not match the resonance wavelength of any pillbox optical resonator. This means that each pillbox optical resonator selectively filters only certain incident wavelengths.

Next, the scattering matrix in FIG. 6 is solved using the same parameters as in FIG. At this time, first, the position delay required to shift the resonance characteristics of the pillbox type optical resonators 11, 12, and 13 as shown in FIG. 7 is calculated.

Calculations for the pillbox type optical resonators 12 and 13 show Δφ ₂ = 4.932 × 10 ⁻⁴ and Δφ ₃ = 9.86, respectively.
It becomes 5 × 10 ^-4 . In practice, it is sufficient to apply a voltage to the electrodes so that the phase delay is generated by these values.

Numerical calculation results are shown in FIGS.
Shown in (C) and (D). These represent the output characteristics at port P-2, port P-5, port P-10, and port P-11 in FIG. 6, respectively. Further, the input wavelengths λ ₁ , λ ₂ , λ ₃ , and λ ₄ are shown in the figure, but from the condition of Δλ = 0.2824 nm, λ ₁ = 0.90, so that the condition of FIG. 7 is satisfied.
01240 μm, λ ₂ = 0.9004770 μm, λ ₃ = 0.9008300
We chose μm, λ ₄ = 0.9011830. As shown in FIG. 7, each figure shows that only one incident wavelength is transmitted and it operates as a wavelength selection filter. Therefore, when the crosstalk at each output port is calculated, output port P-2 and output port P-
5, -10.20 dB, -12.32 dB, -14.07 dB, -2 at output port P-10 and output port P-11, respectively
It will be 8.72 dB.

Here, the incident wavelength interval is calculated to be 5Δλ / 4.
= 0.353 nm, but as is clear from FIG. 7, if the wavelength interval is LΔλ / 4 (L: odd number),
Since similar filter characteristics can be obtained, when selecting the incident wavelength to be actually used, it may be determined in consideration of the oscillation spectrum line width of the laser diode.

(4) Summary An input / output waveguide is attached to a pillbox type optical resonator composed of a thin disk-shaped core, and the wavelength-dependent characteristics of the input / output of the optical power are calculated by eigenvalue calculation based on FE-BPM and mode. Numerical calculations were performed by incorporating the results of the transformation calculation into the scattering matrix to clarify its resonance characteristics. Then, as its application, we proposed an optical filter with a structure in which multiple coupling systems are combined, electrodes are further attached to the pillbox type optical resonator, and each resonance characteristic is shifted by applying a voltage. By numerically calculating the wavelength-dependent characteristics of, it was confirmed that it certainly works as a wavelength selection filter.

[0034]

The present invention has three main points. The first is
As shown in FIG. 1A, by arranging the pillbox type optical resonator and the coupling waveguide for input and output along the outer periphery of the pillbox type optical resonator at a predetermined angle at equal intervals,
This is to improve the controllability of the coupling characteristics and facilitate the design.

Conventionally, a pillbox type optical resonator employing a linear waveguide as a coupling waveguide has been reported, but in this case, it is not uniform over the coupling region, and it is difficult to design to obtain optimum coupling. . According to the method of the present invention, since a uniform bond can be obtained over the bond region, the bond characteristics can be changed by changing the length of the bond portion, which facilitates the design.

The second important point of the present invention is that the resonance wavelength is continuously variable by providing an electrode on a part of the pillbox type optical resonator. As a result, the demultiplexing and multiplexing characteristics can be electrically controlled, the application range can be expanded, and the manufacturing tolerance can be increased. In the absence of electrodes, the resonant wavelength and phase velocity along the perimeter of the resonant mode are determined solely by the refractive index and dimensions of the pillbox optical resonator. By arranging the electrodes, the refractive index of the portion can be changed by utilizing the electro-optic effect, and the resonance wavelength and the phase velocity can be changed.

Further, as a third point of the present invention, as shown in FIG. 3, by connecting a plurality of the above-mentioned wavelength selection filters in cascade, a wavelength selection filter for individually selecting optical wavelengths of multiple channels is realized. That is. Figure 3 shows three wavelength selection filters (11, 12, 13) with two coupled waveguides.
Is an example of a demultiplexing circuit in which four wavelengths of light are input in series and are output to different ports. The characteristics of each individual optical wavelength selection filter can be electrically controlled independently. In the configuration shown in FIG. 3, the three filters may be designed to resonate at only one of the wavelengths respectively entered.

[Brief description of drawings]

FIG. 1A is a plan view of a wavelength selection filter in which a pillbox type optical resonator of the present invention and a coupling waveguide are coupled. FIG. 1B is a model diagram by a scattering matrix of a coupling system of a pillbox type optical resonator and a coupling waveguide.

FIG. 2A is a layout view of electrodes arranged on a non-coupling portion of the wavelength selection filter of the present invention. FIG. 2B is a layout view of the electrodes arranged on the coupling portion of the wavelength selection filter of the present invention.

FIG. 3 is a plan view showing an example of a multi-channel wavelength selection filter in which three wavelength selection filters are cascade-connected.

4A and 4B are output light intensity diagrams of output ports of a port P-2 and a port P-3 of the wavelength selection filter shown in FIG.

5A is a demultiplexing characteristic diagram of the output characteristic of the waveguide 21 of the wavelength selection filter having the configuration of FIG. FIG. 5B is a demultiplexing characteristic diagram of the output characteristic of the waveguide 22 of the wavelength selection filter having the configuration of FIG. FIG. 5C is a demultiplexing characteristic diagram of the output characteristic of the waveguide 23 of the wavelength selection filter having the configuration of FIG. FIG. 5D is a demultiplexing characteristic diagram of the output characteristic of the waveguide 24 of the wavelength selection filter having the configuration of FIG.

6 is a model diagram of a scattering matrix of the multichannel wavelength selective filter of FIG. 3 according to the present invention.

FIG. 7 is a characteristic diagram showing a relationship between an input wavelength and a resonance wavelength in the case of four wavelengths.

[Explanation of symbols]

 1 Pill Box Optical Resonator 2 Input / Output Coupling Waveguide 3 Input / Output Coupling Waveguide 4 Electrode 10 Substrate 11 Pill Box Optical Resonator 12 Pill Box Optical Resonator 13 Pill Box Optical Resonator 21 Waveguide 22 Waveguide 23 Waveguide 24 Waveguide

Continued Front Page (72) Inventor Ryuichi Furuya 226-5 Fukui, Kurashiki-shi, Okayama

Claims (2)

[Claims] 1. A pillbox type optical resonator and a coupling waveguide for input and output are arranged at equal intervals along a circumference of the pillbox type optical resonator, and electrodes are provided on a part thereof. Thus, the wavelength selection filter of the pillbox type optical resonator is characterized in that the resonance wavelength is continuously variable. 2. The wavelength selection filter for a pillbox type optical resonator according to claim 1, wherein a plurality of the wavelength selection filters are connected in cascade.