JP4795289B2 - Optical device apparatus having coupled optical waveguide - Google Patents

Description

  The present invention relates to an optical device device having a coupled optical waveguide using a photonic crystal.

  An artificial crystal in which a spatial periodic refractive index structure is formed in a dielectric is called a photonic crystal, and a phenomenon that prohibits the presence of light in a specific band appears. This corresponds to the band gap for electrons in the solid crystal and is called a photonic band gap. By utilizing this phenomenon, light can be confined strongly in a minute space, and realization of a minute light wave circuit in which many optical elements are integrated is expected.

  The two-dimensional photonic crystal is an optical material having a two-dimensional periodic refractive index distribution. For example, a hexagonal lattice two-dimensional photonic crystal structure in which air holes are arranged in a hexagonal lattice shape as shown in FIG. 1 can be considered. . Here, arbitrary structures are allowed as the unit structure and the crystal lattice. Examples of the unit structure other than the air hole include a triangular hole and a dielectric cylinder. Examples of crystal lattices other than the hexagonal lattice include a tetragonal lattice and a rectangular lattice.

  When a linear defect that disturbs the periodic refractive index distribution is produced in the two-dimensional photonic crystal, light is strongly confined in the defect and propagates in the defect direction. This functions as an optical waveguide.

  Various structures are conceivable as the defect structure that disturbs the periodic refractive index distribution. For example, a method of adding a dielectric (FIG. 2A), a method of removing the dielectric (FIG. 2B), a method of shifting the crystal structure (FIG. 2C), and the like can be mentioned. It is also possible to form a defect structure by combining these simultaneously.

  The waveguide mode of the two-dimensional photonic crystal optical waveguide shows a unique dispersion relationship, and the dispersion relationship can be controlled by modulating the crystal structure. Realization of a dispersion compensation element, an optical delay element, and the like using this feature is expected.

  One example of an optical device device using a two-dimensional photonic crystal is a directional coupler. A general structure using a photonic crystal is shown in FIG. In this case, both ends of a coupled waveguide in which two waveguides are arranged in parallel are connected to two single linear waveguides, respectively.

  The transmission spectrum of a directional coupler inherently shows the characteristic that the Bar direction and the Cross direction change complementarily. However, the transmission spectrum obtained by actual measurement did not show complementary characteristics as shown in FIG.

  FIG. 5A is a magnetic field distribution of light propagating through a conventional branching waveguide, and FIG. 5B is a reflection spectrum that flows backward through the other input waveguide and a transmission spectrum in the Bar and Cross directions at the input end of the coupled waveguide. . Ideally, the intensity of the light that flows back through the other input waveguide is 0, and the transmitted light intensity in the Cross direction at the input end of the coupled waveguide is also 0, but the other input guide is reflected by reflection at the end of the coupled waveguide. Light flows backward to the waveguide (slant single waveguide), and light leakage in the Cross direction is also large. This is because most of the transmitted light is coupled to a mode that is not confined inside the coupling waveguide, as can be seen from the magnetic field distribution.

  FIG. 6 is a photograph of the directional coupler being measured taken from above. Light is scattered upward at both ends of the coupled waveguide and observed as bright spots.

As a method for avoiding light scattering and reflection in the waveguide without including the conventional (bent) bent waveguide, the photonic crystal optical waveguide is gently distorted along a free curve as shown in FIG. 7B. Waveguides have been proposed. This optical waveguide can gently change the traveling direction of the guided light.
This photonic crystal optical waveguide is a premise of the present invention. “In a two-dimensional photonic crystal, a straight line connecting lattice points is distorted along a free curve, and the entire crystal structure is deformed accordingly. The refractive index structure on the curve is removed, and the optical curve is defined as the waveguide having the free curve as the center line of the waveguide.
The photonic crystal optical waveguide is not limited to the air hole hexagonal lattice photonic crystal of FIG. 7, but can be applied to any two-dimensional photonic crystal such as the square lattice of FIG.

Improve the characteristics of the directional coupler by applying the above optical waveguide to a branching waveguide for connecting two photonic crystal single waveguides to a pair of coupled waveguides of a directional coupler Can do. For this purpose, two curved waveguides may be joined as shown in FIG. However, the optical waveguide also loses the translational symmetry of the original photonic crystal, so that there is a problem that the periodic structure of the refractive index is greatly disturbed at the junction and light cannot be guided.
Y. Tanakaet.al, Jpn J. Appl.Phys. 44, 4971 (2005) N. Yamamoto et.al, Opt.Express 14 1223 (2006)

  An object of the present invention is to obtain an optical device device that smoothly shifts a plurality of independent photonic crystal free-curve optical waveguides to an optically coupled state while suppressing reflection and scattering.

Means for solving the problems of the present invention are as follows.
(1) A plurality of independent photonic crystal free-curve optical waveguides can be obtained by modulating the dielectric change structure in a region where the waveguides overlap each other and the periodicity of the refractive index is lost. An optical device having a coupled optical waveguide, wherein the waveguide mode is adiabatically changed from a waveguide mode to a waveguide mode of a coupled waveguide.
(2) For the photonic crystal free-curve optical waveguide on the semiconductor slab in which the air holes are periodically arranged in the tangential direction and the normal direction, by modulating the position and radius of the air holes near the junction, the dielectric The optical device apparatus having the coupled optical waveguide according to (1), wherein the body change structure is modulated.
(3) The coupled optical waveguide according to (1) or (2), wherein two single waveguides and a pair of coupled waveguides are connected using the photonic crystal free curve optical waveguide. Optical device apparatus having.

  According to the present invention, a plurality of independent photonic crystal free-curve optical waveguides can be smoothly transferred to a state where the waveguides are close to each other and optically coupled while suppressing reflection and scattering. For this reason, the waveguides can be joined without degrading the optical characteristics.

Hereinafter, the present invention will be described by way of examples.
An embodiment applied to a branching waveguide connected to a coupling waveguide of a directional coupler from two single waveguides using an adiabatic curved waveguide will be described. In the present invention, arc-shaped adiabatic bent waveguides are joined as shown in FIG. 10 to convert the propagation mode of a single waveguide into the propagation mode of a coupled waveguide without reflection or scattering.
That is, the joint surface between the two bent waveguides A and B is processed as follows according to the way the circles overlap. (a): When the circles A and B are separated, the radius of the approaching circle is increased or the position of the circle is shifted to the joining line side. (b): When the circles A and B approach, the radius of the approaching circle is reduced or the position of the circle is shifted away from the joint line. (c): When A and B circles overlap, the original circular hole is removed and a large circular hole is created at the position of the joining line (shown by a broken line). (d): When the circles A and B are largely overlapped, the original circular hole is removed and a small circular hole is created at the position of the joining line (shown by a broken line). (e): When the circles A and B completely exceed the joint line, the circular hole beyond the joint line is removed.

An example of the modulation method near the joint line is shown below. The y coordinate of the joint line is 0, the coordinates of each air hole is (xc, yc), the radius is rc, and the adiabatic curved waveguide on the upper side of the joint line (y> 0 side)
(1) When yc ≦ −0.5, this circular hole is removed. ((F) in FIG. 10)
(2) When -0.5 ≦ yc <0.25, this circular hole is removed, and a circular hole with a radius yc + rc is created at the position (xc, 0) (on the joining line). ((D), (e) in Fig. 10)
(3) If 0.25 ≤ y <0.5, change the radius of this hole to 2.5rc | yc |. ((B) and (c) in Fig. 10)
For the adiabatic bent waveguide on the lower side of the joint line (y <0), the same modulation is performed so that the reflection symmetry is maintained with respect to the joint line.

  FIG. 10A shows the magnetic field distribution of light propagating through the adiabatic Y-branch waveguide, and FIG. 10B shows the reflection spectrum flowing back through the other input waveguide and the transmission spectrum in the Bar and Cross directions at the input end of the coupled waveguide. . Compared with the result calculated for the conventional branching waveguide (FIG. 5), reflected light is greatly suppressed, and leakage of light in the cross direction at the input end of the coupled waveguide can also be suppressed.

  FIG. 12 shows power spectrum measurement results of the directional coupler using the adiabatic change structure. The structure other than both ends of the coupled waveguide, that is, the structure of the coupled waveguide portion is the same as that used in the measurement of FIG. 7, and the length is also set to 36a. The output intensities in the Bar direction and the Cross direction fluctuate complementarily, and the original characteristics of the directional coupler are correctly obtained.

  FIG. 13 shows an optical buffer circuit fabricated using the branching waveguide shown in FIG. According to the present invention, since all of the conventional sharp bends can be removed, deterioration of optical characteristics due to scattering / reflection in the directional coupler and the ring waveguide can be greatly reduced.

  FIG. 14 shows an SEM image of the manufactured adiabatic branching waveguide. All fabrication uses electron beam lithography technology. The structure of the present invention does not require a special structure or manufacturing process for forming a free-form curve waveguide, and equipment and a manufacturing process for manufacturing a two-dimensional photonic crystal can be used as they are.

Example structure of two-dimensional photonic crystal Example of how to create a two-dimensional photonic crystal line waveguide Conventional directional coupler using curved waveguide Transmission spectrum of a directional coupler using a conventional curved waveguide (a): Magnetic field distribution of light propagating through a conventional Y-branch waveguide, (b): Reflected spectrum reflected at the end of the coupled waveguide and flowing back through the other input waveguide, and at the end of the input-side coupled waveguide Transmission spectra in the Bar and Cross directions Photograph taken from above during measurement of a directional coupler using a conventional curved waveguide Free curve optical waveguide (in case of hexagonal lattice) Free curve optical waveguide (in the case of square lattice) Conceptual diagram of approaching one waveguide end of each of two free-curve waveguides A branching / combining waveguide for coupling waveguides designed by joining two circular free-form optical waveguides FIG. 10 shows a magnetic field distribution of light propagating through a branching waveguide in which the structure of FIG. 10 is introduced; (b): a reflection spectrum reflected at the end of the coupled waveguide and flowing back through the other input waveguide; Transmission spectra in the Bar and Cross directions Transmission spectrum of directional coupler using branching waveguide with the structure of FIG. A directional coupler using a branching waveguide having the structure of FIG. 10 and an optical buffer circuit using a free curve waveguide without a sharp bend SEM photograph of branching waveguide with heat insulation structure

Claims (3)

  Multiple independent photonic crystal free-curve optical waveguides can be used to modulate a single-waveguide waveguide mode by modulating the dielectric change structure in a region where the waveguides overlap and the periodicity of the refractive index is lost. Device device having a coupled optical waveguide, wherein the optical waveguide device is adiabatically changed to a waveguide mode of the coupled waveguide.   For a photonic crystal free-curve optical waveguide on a semiconductor slab in which air holes are periodically arranged in the tangential and normal directions, the dielectric change structure is modulated by modulating the position and radius of the air holes near the junction. The optical device apparatus having a coupled optical waveguide according to claim 1, wherein:   3. The optical device apparatus having a coupled optical waveguide according to claim 1, wherein two single waveguides and a pair of coupled waveguides are connected using the photonic crystal free curve optical waveguide.