JP3876863B2 - Add-drop filter - Google Patents

Description

  The present invention relates to an add / drop filter, and in particular, a photonic crystal that can be used for a basic structure and optical components constituting various optical devices such as lasers and optical integrated circuits used for optical information processing and optical transmission. The present invention relates to an add / drop filter for adding / dropping light of a specific wavelength.

  Since current optical devices perform light confinement with a refractive index, the light confinement region cannot be reduced, so that the element cannot be made small. Further, if a steep bent waveguide is formed in order to increase the degree of integration of elements, scattering loss occurs, so that the optical circuit cannot be miniaturized and integrated, and its size is much larger than that of an electronic device. Therefore, a photonic crystal capable of confining light with a completely different concept from the conventional one is expected as a new light material that can solve the above-mentioned problems.

  A photonic crystal is an artificial multidimensional periodic structure in which a periodicity equivalent to the wavelength of light is formed by two or more types of media having different refractive indexes, and has a light band structure similar to an electron band structure. Have. Therefore, a forbidden band of light (photonic band gap: PBG) appears in the specific structure. When a point defect or line defect that disturbs periodicity is introduced into a photonic crystal having this, it becomes possible to completely confine light in the defect. It functions as a waveguide. Moreover, various functionalities can be realized by combining point defects and line defects.

  FIG. 10 is a simplified diagram of a conventional add / drop filter, in which a cavity is disposed between two waveguides.

  As shown in FIG. 10, the configuration in which the cavities are arranged in the two waveguides functions as a wavelength filter. That is, only when the light input from the port 1 resonates with the cavity, the light is extracted from any one or all of the ports 1 to 4, and the light is extracted only from the port 2 at other wavelengths. . In particular, when the following two conditions are satisfied at the same time, light of a specific wavelength can be extracted from either port 3 or port 4, and light of other wavelengths can be extracted only from port 2. -It is called a drop filter (for example, refer nonpatent literature 1).

  (Condition A) Two modes having the same resonance wavelength exist between two waveguides.

  (Condition B) These modes have a symmetric / antisymmetric field distribution with respect to the symmetry plane shown in FIG.

  A mode that satisfies this condition B is realized by using two types of mode coupling (each having a phase relationship of the same phase and a reverse phase) generated when two independent single mode cavities are brought close to each other. It is known that it can be done. However, since the resonance wavelength of such a coupling mode is always separated into two, the condition A cannot be satisfied. For this reason, several structures have been proposed.

  One of them is a method in which two isolated single modes and a cavity are coupled by a waveguide, and the phase relationship between the cavities is adjusted by the waveguide length between them (see, for example, Non-Patent Document 2). A method of using two different modes in one cavity has also been proposed. However, these methods are not practical because these filter structures used in numerical experiments use photonic crystals that cannot confine light in the thickness direction. Even if an equivalent photonic crystal that can confine light in the thickness direction is realized, the former is to optimize the dispersion and length of the waveguide in order to adjust and change the resonant wavelength of the cavity. The latter requires a search for a structure having two different modes with the same resonance wavelength in one cavity, which requires not only a great amount of time and effort but also allows fine adjustment of these. Therefore, extremely high production accuracy is required.

In order to solve such a problem, there is a method of using one mode in one cavity (for example, see Non-Patent Document 3). By using a cavity with weak light confinement in the thickness direction, it is possible to extract the resonated light out of the two-dimensional plane. Since one mode is used, it is a major feature that strict manufacturing accuracy is not required, and use at a coupling portion between the outside and the optical circuit is expected.
C. Mananolatou et al. IEEE J. Quantum Electron, 35,1322, (1999) S.Fan et al Optics Express.vol.3, (1998) S. Noda et al, Nature 407,608, (2000)

  However, the method of using one mode in one cavity has a problem that a complicated circuit cannot be formed in a two-dimensional plane because light is taken out of the plane.

  The present invention has been made in view of the above points, and has a structure of an add / drop filter based on a photonic crystal that can solve the above-mentioned difficulty of design and production and can constitute an optical circuit in a two-dimensional plane. The purpose is to provide.

The present invention is an add / drop filter having a line defect obtained by removing a part of lattice points of a slab type two-dimensional photonic crystal in a straight line and a point defect,
A slab type two-dimensional photonic crystal with holes in a triangular lattice shape,
Two line defect waveguides obtained by removing the lattice points of the triangular lattice in a straight line;
Between the two line defect waveguides, it has a configuration in which one point defect cavity excluding a row of a plurality of holes is arranged,
Point defects are
The resonance mode is a resonator originating from a waveguide mode that propagates through a linear defect. By using the abnormal dispersion characteristics of the waveguide mode, the dispersion curve is composed of a substantially straight optical waveguide. Compared to the case where a resonator is used, the mode interval has an abnormally narrow band, and thus two modes having antisymmetric field shapes are present at very close frequencies.

  Further, the point defect of the structure of the present invention is constituted by a defect in which a plurality of holes are removed in a row, and the dispersion curve is obtained by utilizing the abnormal dispersion characteristic of the waveguide constituted by removing the holes in a row. A cavity having a band with an unusually narrow mode interval is used compared to the case of using a resonator composed of a substantially straight optical waveguide.

Further, in the present invention , in order to satisfy the single mode condition, the distance between the centers of the holes in one line on both sides of the waveguide formed excluding the line of lattice points is 0.47 times to 0.69 times and 0. 7 to 1.17 times, and deform the hole shape of the outer row of the shifted hole row so that it is extended by the shifted amount, or if the center-to-center distance exceeds 1 time, change the hole shape Not to be a waveguide.

The present invention is directed to a hole to remove the point defect cavities and twelve ~∞ pieces.

Further, according to the present invention , in order not to disturb the structure of the line defect and the point defect, the distance between the two waveguides is set to 4 which is the center distance of the holes in one row on both sides of the waveguide formed by excluding one row of lattice points. Double to ∞ times .

In the present invention , the waveguide uses an oxide cladding or a polymer cladding,
An SIO substrate is used for the photonic crystal .

  In the present invention, a single cavity having a single line-line defect (W1) is terminated shortly between two waveguides, and anomalous dispersion of the waveguide is utilized to provide a two-dimensional in-plane. Thus, an add / drop filter based on a photonic crystal that can constitute an optical circuit can be realized.

  In addition, according to the present invention, the cavity automatically satisfies the conditions necessary for the add / drop filter, and this structure greatly facilitates the design / production of the add / drop filter.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows the configuration of an add / drop filter according to an embodiment of the present invention, which is an add / drop filter combining two line defects 4 and one point defect 1 in a photonic crystal.

  FIG. 4A is a top view of the add / drop filter, and FIG. 4B is a cross-sectional view taken along line AA ′ of FIG.

  The add / drop filter shown in the figure uses a photonic band gap (PBG) having a wide band, and therefore has a triangular lattice shape (lattice constant a) on a semiconductor substrate 6 having a refractive index larger than that of glass or the like. A two-dimensional slab photonic crystal is used as the semiconductor slab 3 with the (air hole 2) opened, and one point defect 1 is arranged between two line defects 4 excluding the lattice points 5 in a straight line. Is.

  This two-dimensional slab photonic crystal is known to have a wider PBG band than other two-dimensional crystals, and is advantageous for application to devices.

The point defect cavity 1 is composed of defects in which several holes are removed in a row, and has a band with an unusually narrow mode interval by utilizing the abnormal dispersion characteristics of a waveguide constructed by removing holes in a row. The cavity is a cavity that adjusts the resonance wavelength and the coupling with the adjacent waveguide by shifting one row of holes on both sides sandwiching the defect in the center direction of the line defect or in the opposite direction. The shift amount at that time is such that the center distance (W _C ) of the holes on both sides of the line defect is 0.7 times the distance (W ₀ ) before the shift so that the transverse mode becomes single in the band to be used. Adjust between 1.17 times. If necessary, the hole shape of one row outside the shifted hole row is deformed so as to be extended by the shifted amount. However, if the center distance of the holes exceeds one time before the shift, the shape of the hole in one row outside the shifted hole row is not deformed.

  Here, the “band with an unusually narrow mode interval” will be described.

The dispersion of the photonic crystal waveguide W1 (W _C = W ₀ ) is a curve as shown by the solid line in FIG. In the frequency band, it is asymptotic to the dotted line in the figure, and in a certain band, it deviates greatly from that (gray area in FIG. 2B). On the other hand, the dispersion of the conventional optical waveguide is a shape close to a straight line, and asymptotically approaches the dotted line in the figure if the size is the same. That is, the gray region in FIG. 2B shows the anomalous dispersion characteristics of the photonic crystal waveguide.

  Here, the mode of the resonator constituted by the waveguide exists at the normalized wave number interval Δk = 1 / (2L). Here, L is the resonator length. Therefore, in the case of a resonator composed of a conventional optical waveguide having a substantially linear dispersion curve, the mode frequency interval (longitudinal mode interval α) is substantially constant as shown in FIG. On the other hand, in the case of a resonator composed of a line defect waveguide (W1), as shown in FIG. 2C, the longitudinal mode interval approaches 0 in an abnormal dispersion region. In this way, by applying the photonic crystal waveguide to the resonator, it is possible to create a “band having an unusually narrow longitudinal mode interval”.

The point defect cavity 1 has 12 or more holes to be removed (12 or more to ∞) in order to narrow the mode interval. Here, the reason why 12 or more holes are removed will be described.

  FIG. 3 shows an example in which the mode interval is narrowed. FIG. 2 (a) corresponds to FIG. 3 (a), and FIG. 2 (c) corresponds to FIG. 3 (b). When the two modes (a) and (c) shown in FIG. 2 overlap as shown in FIG. 3, the above-described conditions A and B are satisfied. It is clear from FIG. 2 that Δk → 0, that is, L → ∞, in order to strictly overlap the two modes. However, since the half width of the mode is finite, the overlap between the two modes does not need to be exact. That is, if the modes can be overlapped to some extent, an add / drop filter can be configured with a resonator having a finite length. The degree of overlap of the modes is determined by the half width of the mode and the mode interval. That is, a narrower half-width mode requires a smaller mode interval, and a wider half-width mode relaxes this requirement. The half width is the widest when the optical confinement of the resonator is the weakest, that is, when the interval between the waveguides 4 sandwiching the resonator is the narrowest. Therefore, the shortest resonator length for functioning as an add / drop filter can be searched under this condition.

FIG. 4 shows the wavelength demultiplexing characteristics when the interval between the waveguides is fixed to the minimum value (4W ₀ ) and resonators having various lengths are arranged therebetween. In the figure, the transmission spectrum calculated using the FDTD method is shown, and the dotted line, the thick solid line, and the thin solid line in FIG. Represents.

  When the resonator length L = 8a (a is a lattice constant), two modes can be confirmed in the resonator. When L = 10a, these modes start to overlap, and when L = 12a, they start to function as an add / drop filter. In other words, the number of air holes in the photonic crystal removed to form the resonator must be “12 or more”. Conversely, the half width is the narrowest when the waveguide interval approaches ∞. At this time, in order to strictly approach the mode interval to 0, L → ∞, that is, the number of air holes of the photonic crystal removed to constitute the resonator must be “∞”.

In addition, two line defects 4 are formed by shifting one line of holes on both sides of the line defect excluding one line of holes toward the center of the line defect, and shifting the hole shape of one line outside the shifted hole line. The structure is formed by stretching by the length of the distance, and the structures of the line defect and the point defect are arranged at a distance of 4 W ₀ or more so as not to collapse each other.

Further, in order to satisfy the single mode condition, the center distance (W) of the shifted hole sandwiching the line defect is adjusted between 0.47 Wo to 0.69 Wo and 0.73 Wo to 1.17 Wo. is doing. However, when the center distance of the hole exceeds one time before the shift, the hole shape of the outer one row of the shifted hole row is not deformed.

  In the present embodiment, a waveguide having a waveguide mode useful for a two-dimensional photonic crystal in an SOI (Silicon On Insulator) structure with an increase in the scale of an optical circuit based on a future photonic crystal. It was adopted.

  The operation principle in the above configuration will be described below.

  The resonance mode of the resonator based on the waveguide is classified into a symmetric mode and an anti-symmetric mode with respect to the symmetry plane shown in FIG. 10, and in order from the mode end of the longer resonance wavelength, The fundamental mode (symmetric), the first order mode (antisymmetric),... Are arranged in this order, and the interval between the longitudinal modes is proportional to the slope of the dispersion curve of the waveguide. That is, in the conventional optical waveguide in which the dispersion curve is drawn as a substantially straight line, the resonance mode interval is substantially constant. On the other hand, since an optical waveguide composed of a photonic crystal has an abnormal dispersion curve, it is possible to create a mode interval that cannot be considered in the past.

  FIG. 5B shows a dispersion curve of the single-line defect (W1) waveguide of the photonic crystal. In the figure, the horizontal axis represents the normalized wave number (the number of light waves per length a), and the vertical axis represents the normalized frequency (lattice constant / wavelength). It can be seen that the waveguide mode in the PBG indicated by the bold line has an extremely small slope in the gray region, and the longitudinal mode interval of the cavity formed by this waveguide is abnormally small. In other words, in this band, the symmetric mode and the antisymmetric mode can always appear at substantially the same wavelength. Therefore, basically, by simply arranging the resonator between the two waveguides, the mode edge can be obtained. In the vicinity (gray area), the above-described conditions A and B can be satisfied.

  In the present invention, this anomalous dispersion band (gray region) is adjusted by changing the width of the waveguide W1.

  FIG. 5 is a diagram in which one row of holes on both sides sandwiching the defect is shifted in the direction of the center of the line defect or in the opposite direction, and FIG. It is shown that the band in which the anomalous dispersion appears can be adjusted by deforming so as to extend only.

However, in order to improve the add / drop efficiency, it is necessary that the cavity has a single transverse mode in this band. Therefore, the width (W _C ) is as shown in FIGS. It is limited to 0.7W ₀ ~1.17W _0. Here, when the center distance of the hole exceeds 1 time before the shift, the hole shape of one row outside the shifted hole row is not deformed.

The coupling efficiency of the cavity mode and the two waveguide modes sandwiching the cavity mode can be adjusted by adjusting the waveguide structure. As shown in FIGS. 6 and 7, the waveguide has one row on both sides sandwiching the line defect except for one row of holes, shifted toward the center of the line defect, and one row outside the shifted hole row. The width (W) is adjusted by extending the shape of the hole by a shift amount, and W = 0.73W _{0 to} 1.17W ₀ (FIG. 6) or W = 0.47W _{0 to} 0. At 69 W ₀ (FIG. 7), the single mode condition (gray area in the figure) for improving the efficiency of add / drop is satisfied.

Here, it should be noted that the waveguide shown in FIG. 7 also functions in a structure using an oxide or polymer having a refractive index of about 1.5 for the cladding layer. In the waveguide W1, when a medium other than air is used for the cladding layer, light leaks to the cladding layer. This light leakage can be easily explained by using a light line (frequency = wave number / refractive index) determined by the refractive index of the cladding. For example, in the case of the W1 waveguide shown in FIG. 5B, most of the waveguide mode exists on the higher frequency side than the light line of the S _i O ₂ cladding. Region than light line high frequency side, it means that the light leaks to the cladding layer, this is, the W1 waveguide, the cause can not be applied cladding layers such as oxides and polymers such as S _i O ₂ It has become. On the other hand, the waveguide shown in FIG. 7 has a wide waveguide mode on the lower frequency side than the S _i O ₂ light line, and it can be seen that an oxide such as S _i O ₂ or a polymer can be applied to the cladding layer. . Usually, the thickness of a two-dimensional photonic crystal made of a semiconductor is about half the wavelength of light and is very thin. The waveguide, which can employ a strong cladding layer other than air, is a great advantage for large-scale integration of optical circuits using photonic crystals.

Further, the coupling efficiency between the cavity mode and the waveguide mode can be adjusted even by the distance between the two waveguides. When the confinement of light in the cavity thickness direction and in-plane direction is equal, the add / drop efficiency is maximized. That is, if the confinement in the thickness direction approaches infinity, the waveguide interval must be set to infinity. On the other hand, when the confinement in the thickness direction is extremely small, it is necessary to narrow the waveguide interval. Since the too narrow interval causes the structure of the waveguide and the cavity to be destroyed, the minimum value of the waveguide interval is 1 means 4W ₀ .

With the above configuration, the structure shown in FIG. 1 can be expected to function as an add / drop filter. However, in order for the cavity used in the structure to have the characteristics of the original W1 waveguide, a resonator length (L) of a certain degree or more is required. Or Here, the waveguide width W = 0.6 W _0, the waveguide spacing 4W _0, sets the width of the cavity in _W C = 1.015W _0, the above phenomenon appears when the extent of the cavity length It was confirmed. Here, the lattice constant is defined as a, the number of air holes removed to form the cavity is defined as N, and the resonator length is defined as L = N × a.

  FIG. 4 shows a two-dimensional simulation result. When L = 4a, this cavity has one resonance mode when the normalized frequency (Z = lattice constant / wavelength) is around 0.255. That is, light input from port 1 is output only from port 2 when the frequency is not the resonance frequency, and light having the same intensity is output to ports 3 and 4 at the resonance frequency. When L = 8a, the resonator length becomes long, so that two resonance modes can be confirmed in the observation band as indicated by arrows, and at L = 10a, these two modes begin to overlap. When L = 12a, light near Z = 0.253 is strongly output from port 4, and the light output from port 3 starts to be kept low. When L = 14a, the output from port 3 becomes extremely small. Only light is output. That is, when the length L = 12a or more, it can be seen that the structure functions as an add / drop filter. In addition, if the optical confinement of the resonator is increased, it is clear that the line width output from the port 4 can be narrowed, and this is achieved by increasing the length of the resonator and increasing the waveguide interval. Ultimately, these values will be infinite.

FIG. 8 shows a schematic diagram of the actually fabricated structure. As shown in FIG. 5, the two-dimensional slab photonic crystal employed in the present invention has PBG at a normalized frequency of 0.25 to 0.3. That is, in order to make PBG appear in the communication wavelength band, the lattice constant may be set to 0.3 to 0.5 μm. In this structure, in order to realize an add / drop operation in the vicinity of a wavelength of 1.55 μm, the lattice constant is set to a = 0.4 μm, and accordingly, the crystal thickness is set to 0.5a (= 0.20 μm). The waveguide spacing was set to 4 Wo (˜2.77 μm) and the cavity width was set to W _C = 0.95 Wo (˜0.66 μm). Since the actual structure is not a two-dimensional structure but a thick three-dimensional structure, it is necessary to consider light confinement in the thickness direction, and the width of the cavity is slightly changed. Further, in order to reliably capture the phenomenon by experiment, a long cavity of L = 30a (= 12 μm) is used.

  FIG. 9 shows the transmission spectrum obtained in the experiment. It can be seen that light entering from the port 1 near the wavelength of 1537 nm is not output from the port 2 but is output from the port 4. In addition, although the output from the port 3 cannot be observed, it is considered that the light having the same intensity as that when the light not having the resonance wavelength is output from the port 2 is output from the port 4 at the resonance wavelength. It is clear that the output from port 3 is suppressed, that is, this structure functions as an add / drop filter.

  Mode coupling between the cavity of the structure and the waveguide can be considered as a kind of directional coupling. Since the field distribution in the vicinity of the mode end of the W1 waveguide spreads widely in the direction of the waveguide sandwiching it, the coupling between the waveguides is strong only in the vicinity of this band. Therefore, this structure functions as a directional coupler with good wavelength selectivity.

In addition, since the group refractive index in this band is abnormally large, the complete coupling length can be shortened. However, if the waveguide W1 is terminated short in order to reduce the size of the device, the end face reflection causes mode coupling in the direction opposite to that of the prior art, causing output from ports other than the port 4. In other words, it is difficult to make the device size extremely small and output light from a specific port only by the idea of placing a waveguide with a slow group velocity between the other two waveguides. Recognize.

  The present invention takes into account the cavity mode and shows that light output is possible only from a specific port even if the device size is reduced by superimposing the two modes.

  The present invention is not limited to the above-described embodiment, and various modifications and applications can be made within the scope of the claims.

  The present invention can be applied to basic structures and optical components that constitute optical devices such as lasers and optical integrated circuits used for optical information processing and optical transmission.

It is a block diagram of the add drop filter in one embodiment of this invention. It is a figure which shows the dispersion characteristic of the waveguide in one embodiment of this invention. It is a figure which shows the example which narrows the mode space | interval of a point defect in one embodiment of this invention. It is the transmission spectrum calculated using FDTD method in one embodiment of this invention. It is a figure which shows the dispersion curve of the photonic crystal waveguide which comprises the cavity in one embodiment of this invention. It is a figure which shows the dispersion curve of the photonic crystal waveguide which comprises the cavity in one embodiment of this invention, or is used for an input-output port. It is a figure which shows the dispersion curve of the photonic crystal waveguide used for the input / output port in one embodiment of this invention. It is the schematic of the add / drop filter actually produced in one embodiment of this invention. It is a figure which shows the transmission spectrum of the add / drop filter obtained by experiment in one embodiment of this invention. It is a simplified diagram of a conventional add / drop filter.

Explanation of symbols

1 point defect cavity 2 air hole 3 semiconductor slab 4 line defect waveguide 5 triangular lattice 6 cladding

Claims (5)

An add / drop filter having a line defect and a point defect obtained by removing a part of lattice points of a slab type two-dimensional photonic crystal in a straight line,
A slab type two-dimensional photonic crystal with holes in a triangular lattice shape,
Two line defect waveguides obtained by removing the lattice points of the triangular lattice in a straight line;
Between the two line defect waveguides, it has a configuration in which one point defect cavity excluding a row of a plurality of holes is arranged,
The point defect is
The resonance mode is a resonator originating from a waveguide mode that propagates through a linear defect. By using the abnormal dispersion characteristics of the waveguide mode, the dispersion curve is composed of a substantially straight optical waveguide. An add / drop filter characterized in that the mode interval has an unusually narrow band than when a resonator is used, and thus two modes having antisymmetric field shapes are present at extremely close frequencies . In order to satisfy the single mode condition, the distance between the centers of the holes in one line on both sides of the waveguide formed excluding the line of lattice points is 0.47 times to 0.69 times and 0 . 7 to 1.17 times, and if the hole shape of one row outside the shifted hole row is deformed so as to be extended by the shifted amount, or if the center-to-center distance exceeds one time , the hole shape is changed to The add / drop filter according to claim 1, wherein the add / drop filter is a non-deformable waveguide . 12 to ∞ holes to remove point defect cavities ,
The add / drop filter according to claim 1. In order not to destroy the structure of the line defect and the point defect, the distance between the two waveguides is 4 to ∞ times the center distance of the holes on both sides of the waveguide formed by excluding one line of lattice points. And
The add / drop filter according to claim 1. The waveguide uses an oxide cladding or a polymer cladding,
Using a SIO substrate for the photonic crystal,
The add / drop filter according to claim 1.

Images