JP2008275708A - Polarization control optical circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarization control optical circuit capable of configuring a polarization separation element in a short refractive index control region. <P>SOLUTION: An effective refractive index control structure 15 is obtained by etching an embedded-type optical waveguide having a core 22 embedded in a clad 21 and filling it with a resin 23. The resin 23, having a refractive index lower than that of the core 22, is used. In a region Z, a groove position is deepest in the core, and a difference is produced between an effective refractive index with respect to TE polarization and that with respect to TM polarization, mainly in this part. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polarization control optical circuit effective for a wavelength multiplexing / demultiplexing circuit, an optical add / drop circuit, an optical switch, a polarization diversity circuit, a coherent reception circuit, or a polarization dispersion compensation circuit.

  In a Mach-Zehnder type interference optical waveguide, it is known that a difference in effective refractive index with respect to TE (Transverse Electric) polarized light and TM (Transverse Magnetic) polarized light is locally provided to separate TE polarized light and TM polarized light (for example, , See Patent Document 1).

  FIG. 8 is a diagram showing the configuration of the Mach-Zehnder type interference optical waveguide of Conventional Example 1. In FIG. The Mach-Zehnder type interference optical waveguide includes a waveguide 81 as an input port 1, a waveguide 82 as an input port 2, a 2: 2 optical coupler 83, a waveguide 84 as a first arm of a Mach-Zehnder interference system, stress Giving waveguide 85, waveguide 86, waveguide 87 as the second arm of the Mach-Zehnder interference system, 2: 2 optical coupler 88, waveguide 89 as output port 1, and waveguide as output port 2 90. The stress-applying waveguide 85 can apply a stress to the optical waveguide, apply stress to the optical waveguide, and use the photoelastic effect to make a difference in effective refractive index with respect to TE light and TM light. By adjusting the magnitude of the stress and the length of the stress application portion, there is no optical path length difference in TE light propagating through the first arm and the second arm, and the optical path length difference is half wavelength for TM light. Can be set to be In this case, the TE light incident from the input port 1 is output from the output port 2, and the TM light incident from the input port 1 is output from the output port 1. That is, it operates as a polarization separation element.

FIG. 9 is a diagram illustrating a configuration of a Mach-Zehnder interference optical waveguide according to the second conventional example. Conventional example 2 is provided with a region 91 in which the waveguide core width is widened in place of the stress-applying waveguide 85 of conventional example 1, and locally makes a difference in effective refractive index with respect to TE polarized light and TM polarized light. The function is the same as in Conventional Example 1.
JP 2006-333139 A (FIG. 5)

However, in these methods, the effective refractive index difference with respect to TE light and TM light is small, and the quartz waveguide requires at most about 5 × 10 ⁻⁵ and requires a refractive index control region of 10 mm or more. The dimensions increase.

  In view of the above problems, an object of the present invention is to provide a polarization control optical circuit capable of constituting a polarization separation element with a short refractive index control region.

  The polarization control optical circuit according to the present invention includes an optical waveguide portion, which is a normal optical waveguide, in a buried optical waveguide having a core embedded in a cladding, and a partial region of the optical waveguide portion. It is characterized by comprising an effective refractive index control structure part which is etched with a part of an adjacent clad on both sides and is an area where the effective refractive indexes of TE polarized light and TM polarized light are different.

  Further, the etched portion is filled with resin or low-melting glass, or the dielectric is filled by vapor deposition, sputtering, or chemical vapor deposition, so that TE polarized light and TM polarized light can be obtained. The effective refractive index can be varied greatly.

  In addition, the temperature dependence of the optical waveguide portion and the effective refractive index control structure portion can be offset, thereby making the temperature independent.

  Further, the TE-polarized light and the TM-polarized light can be separated because the difference in optical path length between the TE-polarized light and the TM-polarized light in the effective refractive index control structure is an integral multiple or a half-integer multiple of the wavelength used.

  Further, since there are a plurality of effective refractive index control structures, the loss of each waveguide can be made the same.

The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows.
(1) It is possible to realize an effective refractive index control structure in which effective refractive indexes for TE polarized light and TM polarized light are greatly different.
(2) It is possible to control the optical path length for TE polarized light and TM polarized light by changing the refractive index of the resin or the length of the effective refractive index control structure.
(3) It is possible to configure a polarization separation element, a polarization independent element, and the like.
(4) Since the effective refractive index difference with respect to TE polarized light and TM polarized light is large, it is possible to make the device compact.
(5) By changing the refractive index of the resin or the length of the effective refractive index control structure, it is possible to connect to a normal waveguide and make the entire optical path length temperature independent.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In all the drawings for explaining the embodiments, parts having the same function are given the same reference numerals, and repeated explanation thereof is omitted. The embodiment will be described on the premise that quartz is used for the optical circuit material, but the same configuration can be applied to other material systems.

  FIG. 1 is a diagram illustrating a configuration of a polarization control optical circuit according to a first embodiment of the present invention. Here, an example applied to a Mach-Zehnder interference optical waveguide is shown. The Mach-Zehnder interference optical waveguide includes a waveguide 11 as an input port 1, a waveguide 12 as an input port 2, a 2: 2 optical coupler 13, a waveguide 14 as a first arm of a Mach-Zehnder interference system, The effective refractive index control structure 15, the waveguide 16, the waveguide 17 that is the second arm of the Mach-Zehnder interference system, the 2: 2 optical coupler 18, the waveguide 19 that is the output port 1, and the output port 2. It consists of a waveguide 20. An effective refractive index control structure 15 is provided in place of the stress-applying waveguide 85 of Conventional Example 1.

  FIG. 2 is an enlarged view and a cross-sectional view of the effective refractive index control structure. 2A is an enlarged view of the effective refractive index control structure, FIG. 2B is a BB sectional view thereof, and FIG. 2C is a CC sectional view thereof. The effective refractive index control structure 15 is a structure in which a buried optical waveguide having a shape in which a core 22 is buried in a clad 21 is etched and filled with a resin 23. In the region X, the clad 21 at a position sufficiently separated from the core 22 is etched, in the region Y, the etching is gradually performed on a part of both sides of the core 22, and in the region Z, both sides of the core 22 are arranged on one side of the adjacent clad 21. Etching is performed together with the portion, and the resulting groove is filled with resin 23. A resin 23 having a refractive index smaller than that of the core 22 is used. For the quartz waveguide, a material made of a fluorine-based resin having a refractive index lower than that of quartz is commercially available. The region X is a region where the groove is sufficiently separated from the core 22 and the groove does not affect the light propagation. The region Y is a transition region in which the light propagation mode changes because the groove position gradually approaches the core and further enters the core 22. The region Z is a region where the groove position enters most inside the core, and the effective refractive index for TE polarized light and TM polarized light is different mainly in this region.

FIG. 3 is a diagram illustrating a calculation example of the effective refractive index. In this calculation, a groove is formed in a quartz waveguide having a relative refractive index difference of 0.75% and a core diameter of 6 μm, and the core width in the region Z is assumed to be 3 μm. It can be seen that the refractive index difference between TE light and TM light is about 1 × 10 ⁻³ . That is, in order to obtain an optical path length difference of about a half wavelength, the length of the region Z may be about 500 μm. Even if the region X and the region Y are included, an optical path length difference of about half a wavelength can be obtained with a total length of about 1 mm, which is 1/10 or less compared to the prior art. If this structure is used, the optical path lengths of the first arm (14, 15, 16) and the second arm (17) in FIG. 1 are made equal to the TE light and different from the TM light by a half wavelength. It is possible to In this case, the TE light incident from the input port 1 is output from the output port 2, and the TM light incident from the input port 1 is output from the output port 1. That is, it operates as a polarization separation element. Of course, conversely, if the optical path lengths of the first arm and the second arm are made equal to the TM light and different from the TE light by a half wavelength, the TE light incident from the input port 1 is output. Needless to say, the TM light output from the port 1 and incident from the input port 1 is output from the output port 2. The optical path length difference between the first arm and the second arm is an integral multiple of the wavelength for one polarization, and the optical path length difference is an odd multiple of a half wavelength (half an integral multiple of the wavelength) for the other polarization. However, it goes without saying that the same operation is performed. Since the effective refractive index control structure 15 has a slight loss, it goes without saying that the extinction ratio is increased by adjusting the branching ratio to the two arms and matching the incident light intensity to the optical coupler to be combined. . In addition, it is possible to make the polarization independent by making the optical path length difference between the arms equal for both polarizations, and TE polarization and TM polarization at an arbitrary ratio by appropriately setting the optical path length difference. Needless to say, you can combine and demultiplex.

  In this embodiment, the effective refractive index control structure is shown in which the groove is filled with resin. However, low melting point glass is filled, or a dielectric is filled by vapor deposition, sputtering, or chemical vapor deposition. It is also possible to do. Further, if the element is packaged in a dust-free state, the same effect can be obtained without filling the groove.

  FIG. 4 is a diagram illustrating a configuration of a polarization control optical circuit according to the second embodiment of the present invention. The Mach-Zehnder type interference optical waveguide includes a waveguide 41 as an input port 1, a waveguide 42 as an input port 2, a 2: 2 optical coupler 43, a waveguide 44 as a first arm of a Mach-Zehnder interference system, Effective refractive index control structure 45a, waveguide 46, waveguide 47 which is the second arm of the Mach-Zehnder interference system, effective refractive index control structure 45b, waveguide 48, 2: 2 optical coupler 49, output port 1 And a waveguide 51 which is the output port 2. The difference from the first embodiment is that an effective refractive index control structure is provided on both arms of the interference system. In order to operate as a polarization separation element, the optical path length difference in both arms for one polarized light is an integral multiple of the wavelength, and the optical path length difference in both arms for the other polarized light is an integral multiple of a half wavelength. Therefore, an effective refractive index control structure with a different length filled with the same resin may be provided. Alternatively, it goes without saying that the lengths of the effective refractive index control structures may be equal, or resins having different refractive indexes may be filled in 45a and 45b. In this configuration, since the same structure is provided in both arms, the loss of both arms is almost equal, and there is an advantage that the extinction ratio of polarization separation is increased without adjusting the branching ratio to the two arms. Further, if resins having different refractive indexes are used for the two effective refractive index control structures, it is not necessary to adjust the absolute value of the refractive index of the resin, which is generally difficult, and the refraction of the two resins, which is generally easy Since the characteristic can be determined by the relative difference in the rate, there is an advantage that the refractive index of the resin can be easily adjusted.

The third embodiment of the present invention is an effective refractive index control structure that controls the temperature dependence of the optical path length or makes the temperature independent. FIG. 5 is a diagram in which the curve of FIG. 3 is drawn with a tangent line, and shows that the equivalent refractive index of the waveguide changes depending on the refractive index inside the groove, and the slope of the tangent line is about 0.14. It is. FIG. 6 shows a waveguide in which an effective refractive index control structure and a straight waveguide are connected in series. Here, the length of the effective refractive index control structure is L1, the resin refractive index is nr, and the temperature dependence of the resin refractive index is nr (T) = nro + (dnr / dT) T (1)
Assume that In addition, since the refractive index change of quartz constituting the effective refractive index control structure is negligible at 1/20 or less compared to the refractive index change of the resin, the temperature change coefficient of the refractive index of the effective refractive index control structure is Can be approximated.

dnc / dT = α (dnr / dT) (2)
Here, nc is the effective refractive index of the effective refractive index control structure, and α is the tangential slope of FIG. The length of the straight waveguide is L2, and its effective refractive index is ns. At this time, if the length is determined as follows, the temperature dependence of the optical path length can be made zero. For simplicity, the length of the transition region is ignored.

α (dnr / dT) L1 + (dns / dT) (L2−L1) = 0 (3)
For example, in the case of a quartz waveguide, the temperature dependence of the effective refractive index of the waveguide is 1.0 × 10 ⁻⁵ . The temperature dependency of the epoxy resin is −2 × 10 ⁻⁴ . Using the value of FIG. 5, α = 0.14,
-3.8L1 + L2 = 0 (4)
If the condition is satisfied, the temperature dependence of the optical path length becomes zero. If each arm length of the polarization beam splitting element shown in Embodiment 2 is adjusted so as to satisfy this condition, it is possible to obtain a temperature-independent characteristic.

  FIG. 7 is a diagram showing a configuration of a polarization control optical circuit according to the fourth embodiment of the present invention. Here, an example applied to an arrayed waveguide diffraction grating is shown. The arrayed waveguide diffraction grating includes an input waveguide 71, a slab waveguide 72, an arrayed waveguide 73, an effective refractive index control structure 74, an arrayed waveguide 75, a slab waveguide 76, and an output waveguide 77. If the third embodiment is applied to the arrayed waveguide 73, the effective refractive index control structure 74, and the arrayed waveguide 75 to satisfy the expression (4), it is possible to make the temperature independent. Furthermore, polarization independence can be realized at the same time by setting the optical path length difference for TE light and TM light to be an integral multiple of the wavelength for each arrayed waveguide.

  In addition, this invention is not limited to the said Example. The present invention can be applied to a wavelength multiplexing / demultiplexing circuit, an optical add / drop circuit, an optical switch, a polarization diversity circuit, a coherent reception circuit, a polarization dispersion compensation circuit, or the like.

  By applying it to a wavelength multiplexing / demultiplexing circuit, it is possible to realize temperature independence and polarization independence at the same time, which is useful for an apparatus for wavelength division multiplexing communication.

  By applying it to an optical add / drop circuit, it is possible to realize temperature independence and polarization independence at the same time, which is useful for an optical cross-connect device.

  In an optical switch using the thermo-optic effect, the temperature in the vicinity of the optical switch rises, but a stable optical circuit can be configured by applying a temperature-independent waveguide to the surroundings.

  The use of the polarization diversity circuit according to the present invention is effective in making polarization-independent components and circuits independent of polarization.

  Further, the coherent reception circuit and the polarization dispersion compensation circuit require optical processing for each polarization, and the polarization diversity circuit according to the present invention is effective.

It is a figure which shows the structure of the polarization control optical circuit by Example 1 of this invention. It is the enlarged view and sectional drawing of an effective refractive index control structure. It is a figure which shows the example of calculation of an effective refractive index. It is a figure which shows the structure of the polarization control optical circuit by Example 2 of this invention. It is the figure which pulled the tangent to the curve of FIG. It is a figure which shows the waveguide which connected the effective refractive index control structure and the linear waveguide in the column. It is a figure which shows the structure of the polarization control optical circuit by Example 4 of this invention. It is a figure which shows the structure of the Mach-Zehnder type | mold interference optical waveguide of the prior art example 1. FIG. It is a figure which shows the structure of the Mach-Zehnder type | mold interference optical waveguide of the prior art example 2. FIG.

Explanation of symbols

11, 41, 81 Waveguide 12, 42, 82 Waveguide 13, 43, 83 2: 2 Optical coupler 14, 44, 84 Waveguide 15, 45a, 45b Effective refractive index control structure 16, 46, 86 Waveguide 17 , 47, 48, 87 Waveguide 18, 49, 88 2: 2 optical coupler 19, 50, 89 Waveguide 20, 51, 90 Waveguide 21 Clad 22 Core 23 Resin 71 Input waveguide 72 Slab waveguide 73 Array conductor Waveguide 74 Effective refractive index control structure 75 Array waveguide 76 Slab waveguide 77 Output waveguide 85 Stress application waveguide 91 Region

Claims (5)

In an embedded optical waveguide with a core embedded in the cladding,
An optical waveguide part which is a normal optical waveguide;
An effective refractive index control structure part which is a part of the optical waveguide part and is etched together with a part of the adjacent clad on both sides of the core, and the effective refractive index of TE polarized light and TM polarized light are different from each other; A polarization control optical circuit comprising:   2. The polarized wave according to claim 1, wherein the etched portion is filled with resin or low-melting glass, or a dielectric is filled by vapor deposition, sputtering, or chemical vapor deposition. Control light circuit.   3. The polarization control optical circuit according to claim 2, wherein the optical waveguide section and the effective refractive index control structure section cancel each other in temperature.   4. The polarization control optical circuit according to claim 1, wherein the optical path length difference between the TE polarized light and the TM polarized light in the effective refractive index control structure is an integral multiple or a half integral multiple of the wavelength used. 5. 5. The polarization control optical circuit according to claim 2, wherein there are a plurality of effective refractive index control structures.

Images