JP2003050324A - Array waveguide type optical multiplexer/demultiplexer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease the reflection in an array waveguide type optical multiplexer/demultiplexer. SOLUTION: In the array waveguide type optical multiplexer/demultiplexer in which a plurality of input waveguides 2, a first slab waveguide 3, a plurality of waveguides 4 having different lengths, a second slab waveguide 5, a plurality of output waveguides 6 are connected in this order, the plurality of waveguides 4 having different lengths are so arranged that the middle points get lined up and a wavelength plate 8 is inserted in a groove 7 provided in the vicinity of the straight line connecting the middle points, the groove 7 is slightly inclined with respect to a symmetry line 12 perpendicular to the plurality of waveguides 4 having different lengths, thus the dependence of a passing light on wavelength is dissolved.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an array waveguide type optical multiplexer / demultiplexer used in a wavelength division multiplexing optical transmission system. 2. Description of the Related Art In a wavelength division multiplexing transmission system, a plurality of signal lights of different wavelengths are multiplexed and transmitted on one optical fiber, so that lights of different wavelengths are multiplexed or wavelength multiplexed. An optical multiplexer / demultiplexer that splits light is an important device. Also in this optical multiplexer / demultiplexer, it is desired that the return loss is 40 dB or more, like other optical communication devices. FIG. 2 schematically shows an array waveguide type optical multiplexer / demultiplexer. As shown in FIG. 1, on a substrate 1, an input waveguide 2, a first slab waveguide 3, an array waveguide diffraction grating 4,
A second slab waveguide 5 and an output waveguide 6 have been made. At the center of the arrayed waveguide diffraction grating 4, a groove 7 is dug in the center of the waveguide constituting the arrayed waveguide diffraction grating 4 in order to eliminate polarization dependence, and a wave plate 8 is inserted into the groove 7. It is fixed with adhesive. [0004] Since the refractive indices of the three materials of the waveguide, the wave plate 8 and the adhesive do not completely match, reflection occurs at the groove 7. In addition, there is also reflection caused by a damaged layer on the groove wall surface when the groove 7 is dug. Hereinafter, these are collectively called reflection from the wave plate. Conventionally, as a method of reducing the influence of the reflection from the wave plate, not only a method of matching the waveguide, the wave plate, and the adhesive but also a method of inclining the groove 7 with respect to the waveguide, not at a right angle. Two are known. FIGS. 4A and 4B show the details. FIG. 3A shows a groove 7 which crosses the waveguide constituting the arrayed waveguide diffraction grating 4 at an inclination of 8 degrees in the horizontal direction.
A wave plate 8 is inserted into the groove 7. FIG.
A groove 7 that crosses the waveguide constituting the arrayed waveguide diffraction grating 4 at an inclination of 8 degrees in the vertical direction is dug, and a wavelength plate 8 is inserted into the groove 7. [0006] In any of the methods shown in FIGS.
By providing the groove 7 with an inclination of 8 degrees, reflected light does not enter the waveguide, and a large return loss can be obtained (Japanese Patent Application No. 3-2903). [0007] However, FIG.
The methods shown in (a) and (b) have the following problems. (1) As shown in FIG. 4A, in the method of digging the groove 7 crossing the arrayed waveguide diffraction grating 4 obliquely with respect to the horizontal direction, the groove 7 is provided with a large inclination. The effect of eliminating certain polarization dependence (Japanese Patent Application No. 3-2903) is reduced. (2) As shown in FIG. 4B, the method of digging the groove 7 crossing the arrayed waveguide diffraction grating 4 obliquely with respect to the vertical direction requires a complicated processing procedure and is most suitable for mass production. Not a way. The present invention has been made in view of the above related art, and has as its object to reduce the reflection of an arrayed waveguide optical multiplexer / demultiplexer. In order to achieve the above object, a configuration of the present invention comprises a plurality of input waveguides, a first slab waveguide, a plurality of waveguides having different lengths, 2 slab waveguides and M output waveguides (M is a natural number) are connected in this order, and the midpoints of the plurality of waveguides having different lengths are arranged in a straight line. An array waveguide type optical multiplexer / demultiplexer in which a wavelength plate is inserted in a groove traversing the plurality of waveguides having different lengths provided in the vicinity of a straight line connecting the grooves, wherein the grooves have different lengths. The angle formed between the direction traversing the waveguide and the plurality of waveguides having different lengths is (π / 2) + δθ (radian) or (π / 2) -δ
θ (radian), and the distance d between the waveguides at the connecting portion between the plurality of waveguides having different lengths and the slab waveguide is different from each other. The distance between the waveguides is D, the distance between the input waveguides at the connection with the first slab waveguide is Δx, the radius of curvature of the first slab waveguide is f, and the effective refractive index of the slab waveguide is When n _S and the effective refractive index of the plurality of waveguides having different lengths are expressed as n _C , the δθ satisfies the following expression and is smaller than 8 °. ## EQU2 ## In the array waveguide type optical multiplexer / demultiplexer using the above means, when the reflected light from the wave plate returns to the first slab waveguide, the converging position is determined by the input waveguide end. Does not. Therefore, since the reflected light from the reflector does not enter the input waveguide, a large return loss can be obtained. Embodiments of the present invention will be described below in detail with reference to the embodiments shown in the drawings. 1 and 2 show an array waveguide type optical multiplexer / demultiplexer according to an embodiment of the present invention. As shown in FIG. 2, an input waveguide 2, a first slab waveguide 3, an array waveguide diffraction grating 4, a second slab waveguide 5, and an output waveguide 6 are formed on a substrate 1. . The waveguide is a quartz-based glass waveguide formed by flame deposition and reactive ion etching. At the center of the arrayed waveguide diffraction grating 4, a groove 7 is dug in the center of the waveguide constituting the arrayed waveguide diffraction grating 4 in order to eliminate polarization dependence, and a wave plate 8 is inserted into the groove 7. It is fixed with adhesive. As shown in FIG. 3, the grooves 7 are not perpendicular to the waveguides constituting the array waveguide diffraction grating 4 but are slightly inclined in the horizontal direction in order to reduce the influence of the reflection from the wave plate. ing. The basic operation of the optical multiplexer / demultiplexer according to this embodiment will be described with reference to FIG. FIG. 1 is an enlarged view showing the vicinity of the first slab waveguide 3 and the second slab waveguide 5.
As shown in the drawing, assuming that an optical fiber (not shown) is connected to the i-th input waveguide 2 and wavelength-multiplexed light is input, the input light 9 passes through the i-th input waveguide 2. After propagating and reaching the first slab waveguide 3, it spreads by diffraction in the first slab waveguide 3 and enters a plurality of waveguides constituting the array waveguide diffraction grating 4. Due to the phase delay effect in the array waveguide diffraction grating 4, the output waveguide 6 to which light is focused differs depending on the wavelength. That is, the wavelength multiplexed light is extracted from the output waveguides 6 which are different for each wavelength. Further, the input light 9 propagates through the array waveguide diffraction grating 4 and is then condensed by the second slab waveguide 5, and enters the j-th output waveguide 6 as output light 11. Here, in order to explain light propagation in detail, an angle is defined as shown in FIG. The i-th input waveguide 2 is in the direction of θ _i , and the j-th output waveguide 6 is in the direction of θ _j
In the direction of. Pitch of array waveguide diffraction grating 4 (interval of waveguide on tangent to slab waveguide)
Let d be the optical path length difference of the array waveguide diffraction grating 4 and ΔL be the wavelength λ _ij of the light input from the i-th input waveguide 2 and output to the j-th output waveguide 6. Satisfy the relationship of the formulas (references, published by IEEE, Journal of Light Tec
hnology, vol.13, pp447-455). N _S dθ _i + n _C ΔL + n _S dθ _j = mλ _ij (1) However, since the diffraction angle is small, it was approximated as sin θ ≒ θ. n _S
Is the effective refractive index of the slab waveguides 3 and 5, n _C is the effective refractive index of the channel waveguide forming the array waveguide diffraction grating 4, and m is the diffraction order. As shown in FIG. 1, if the angle interval between the input waveguide 2 and the output waveguide 6 is Δθ, the i-th
The directions of the second input waveguide 2 and the j-th output waveguide 6 are represented by the following equations. Θ _i = Δθ · i (2) θ _j = Δθ · j (3) From equations (1), (2) and (3), the light enters from the i-th input waveguide 2 and enters the j-th input waveguide 2. The wavelength of the light output to the output waveguide 6 is given by the following equation. λ _ij = [n _S dΔθ (i + j) + n _C ΔL] ÷ m (4) On the other hand, the reflected light 11 from the reflector 8 is
And the light is condensed at the connection between the input waveguide 2 and the first slab waveguide 3 in the same manner as the output light 10 in the second slab waveguide 5. As shown in FIG.
Are formed so as to traverse the waveguides constituting the arrayed waveguide diffraction grating 4 and are perpendicular to the waveguide.
And the angle δθ = (Δxd / 4fD) · (n _S / n _C ). Here, f is the radius of curvature of the first slab waveguide 3. Since the wave plate 8 is inserted into the groove 7,
The intersection position between each waveguide and the wave plate 8 is shifted by D · δθ for each waveguide. Therefore, for the reflected light, the optical path length difference between the waveguides is n _C ΔL + 2 n _C D D δθ. Therefore, the angle θ _i ′ of the condensing point of the reflected light 11 satisfies the following relationship. n _S dθ _i + n _C ΔL + n _S Δxd / 2f + n _S dθ _i ′ = mλ _ij (5) The third term sets the wavelength plate 8 to (Δxd / 4fD) ·
The effect of tilting by (n _S / n _C ) (radian) is shown.
By substituting equations (2) and (4) into this equation, the following equation is obtained. θ _i ′ = (j − ／) Δθ (6) Therefore, the reflected light 11 is converged between the input waveguides 2 and does not enter the input waveguide 2. That is, although there is certainly reflection from the wavelength plate 8, the reflected light 11 does not return to the input waveguide 2, so that there is no reflected return light in the entire optical multiplexer / demultiplexer. A description will be given of specific numerical examples. Generally, in wavelength division multiplex communication, the wavelength interval is about 0.8 nm.
As a design example of an arrayed waveguide type optical multiplexer / demultiplexer corresponding to this, Δx = 25 μm, f = 9400 μm, d = 25 μm
m, D = 30 μm, n _S = 1.445, n _C = 1.444
Is assumed. At this time, in this embodiment, the angle δθ of the groove 7 is
Is (Δxd / 4fD) · (n _S / n _C ) = 5.5 × 10
^-4 (radians), that is, 0.03 degrees. As described above, in the present embodiment, the angle of the groove 7 can be reduced by orders of magnitude as compared with the conventional angle of the groove 7 of 8 degrees. As described above, in the conventional anti-reflection method, the groove 7 has an angle of 8 degrees so that the reflected light from the wave plate 8 does not enter the waveguide. , The polarization dependence of the light transmitted through the array waveguide diffraction grating 4 appears. In this case, there is no point in inserting the wave plate 8 for the purpose of eliminating the polarization dependence. On the other hand, in the present invention as in the present embodiment, the inclination of the groove 7 is only 0.03 degrees, and does not affect the operation of eliminating the polarization dependence. Although the reflected light 11 from the wave plate 8 returns to the waveguide, the phase of the reflected light 11 is shifted between the waveguides.
Deviate. By utilizing this, the intensity of the reflected return light is reduced. In the above embodiment, the position of the reflected return light from the wavelength plate 8 is designed to be shifted by half the input waveguide interval, but the present invention is not limited to this.
The same effect can be obtained even if the interval is shifted by N + 0.5 (N is an integer). Therefore, in general, the condition to be satisfied by δθ is expressed by the following equation. (Equation 3) (N is an integer) When the number of the output waveguides 6 is M (M is a natural number), the reflected light from the wave plate 8 is input even if the following expression is satisfied instead of the above expression. A similar effect can be obtained because the light does not return to the waveguide 2. [Equation 4] Since δθ is a very small value, it is preferable to form the marker 13 as a line indicating the position at which the groove 7 is to be formed. To this end, the line is written in advance on a photomask used for waveguide patterning. It is good to add. Further, in the above-described embodiment, the quartz glass waveguide is used, but the present invention can be similarly applied to a waveguide made of another material. As described above in detail with reference to the embodiments, in the present invention, by making the direction of the groove slightly inclined, the reflected light is prevented from returning to the input waveguide. Can be. Therefore, according to the present invention, an array waveguide type optical multiplexer / demultiplexer having no reflected return light can be provided at low cost, so that a great contribution to wavelength division multiplexing optical transmission can be expected. Of course, since the reflection from the reflector may exist, the above-described refractive index matching is not required.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged view near a slab waveguide of an arrayed waveguide grating optical multiplexer / demultiplexer according to one embodiment of the present invention. FIG. 2 is an overall schematic diagram of an arrayed waveguide grating optical multiplexer / demultiplexer. FIG. 3 is an enlarged view of the vicinity of a wavelength plate of the arrayed waveguide grating optical multiplexer / demultiplexer according to one embodiment of the present invention. FIG. 4 is an enlarged view of the vicinity of a slab waveguide of an arrayed waveguide grating optical multiplexer / demultiplexer according to the related art, and FIG.
The figure shows an image with an inclination of 8 degrees in the horizontal direction, and the figure (b) shows an image with an inclination of 8 degrees in the vertical direction. DESCRIPTION OF SYMBOLS 1 Substrate 2 Input waveguide 3 First slab waveguide 4 Array waveguide diffraction grating 5 Second slab waveguide 6 Output waveguide 7 Groove 8 Wave plate 9 Input light 10 Output light 11 Reflected light 12 Symmetry line 13 marker

Claims (1)

Claims: 1. A plurality of input waveguides, a first slab waveguide, a plurality of waveguides having different lengths, a second slab waveguide, and M output waveguides. (Where M is a natural number) are connected in this order, the midpoints of the plurality of waveguides having different lengths are arranged in a straight line, and the lengths of the plurality of lengths provided near a straight line connecting the midpoints are arranged. In an arrayed waveguide type optical multiplexer / demultiplexer having a wavelength plate inserted in a groove crossing different waveguides, a direction in which the groove traverses the plurality of different length waveguides and a plurality of different length waveguides are provided. Angle (π / 2)
+ Δθ (radian) or (π / 2) -δθ (radian), where d is the waveguide interval at the connection between the plurality of waveguides having different lengths and the slab waveguide, and d is the portion intersecting the groove. The distance between the plurality of waveguides having different lengths is D, the distance between the input waveguides at the connection with the first slab waveguide is Δx, the radius of curvature of the first slab waveguide is f,
When the effective refractive index of the slab waveguide is represented by n _S , and the effective refractive indices of the plurality of waveguides having different lengths are represented by n _C , the δθ satisfies the following expression, and is smaller than 8 °. Array type optical multiplexer / demultiplexer. (Equation 1)