JPH01222216A - Waveguide type polarization plane controller - Google Patents

Abstract

PURPOSE:To facilitate an axis aligning operation process and to realize high accuracy and small size by separating received light which is made incident on a three-dimensional waveguide in two orthogonal polarization directions, and rotating the plane of polarization of one separated light beams by 90 deg. and multiplexing it with the other light beam in in-phase relation. CONSTITUTION:A waveguide type polarization beam splitter 20, a mode converting element 30, a phase shifter 40, and an optical coupler 50 are provided on a substrate 10 which has optical waveguides 11 and 12. Then the incident light is separated into two orthogonal polarization-directional components, which are propagated in the different optical waveguides. Further, the plane of polarization is rotated by 90 deg. in at least one of those optical waveguides 11 and 12 and the phase of the waveguide propagated light is shifted in one of the optical waveguides 11 and 12. Further, a coupler coupling quantity and the shift quantity of the phase shifter 40 are varied so that the output light intensity is maximum. Consequently, the axis aligning process is facilitated and the high accuracy and size reduction are obtained.

Description

Detailed Description of the Invention [Objective of the Invention 1 (Field of Industrial Application) The present invention relates to optical integrated circuits used in optical fiber sensors and optical coherent communications, and particularly relates to waveguide type polarization control devices and the like. The present invention relates to a waveguide type polarizing beam splitter to be used.

(Prior Art) In optical heterodyne and optical homodyne receivers used in optical fiber sensors and coherent communications, it is important to control the plane of polarization of received light in order to improve system performance. In other words, the polarization direction of the signal light propagating through the optical fiber changes irregularly due to the twisting and bending of the optical fiber during installation, as well as at the connection point of the optical fiber. It is difficult to completely match the plane of polarization with the If the planes of polarization of the received light and the local light do not match, many problems will occur, such as a decrease in detection efficiency, SN, and light reception level in the receiver.

The method to recommend matching the polarization planes of the receiving destination and the local light is as follows:
The following methods ■ to ■ have been proposed.

■ A method that uses a polarization-maintaining optical fiber for the transmission line itself.

■ A method of matching the polarization plane of the local oscillation semiconductor laser light used on the receiver side to the polarization plane of the received light.

■ The plane of polarization of the received light is divided into two using a polarizing beam splitter.
A method in which the signal is divided into two directions, each mixed with a local oscillation semiconductor laser, and converted into an electrical signal by a separate PD, which is an optical receiver.

However, these methods have the following problems. That is, in the method (2) using a polarization constant optical fiber, in principle there is no need to control the plane of polarization on the receiver side, but it is necessary to match the plane of polarization at the optical fiber connection point. Therefore, the plane of polarization must be controlled manually,
There is a problem in that careful polarization plane adjustment work is required for each receiving device.

In addition, by a method using a waveguide type element, a phase modulation element and T
There is also a method of controlling the polarization plane of the receiving destination using two E/TM mode conversion elements, but in this case, the polarization plane is controlled with one optical waveguide, so it is difficult to control the polarization plane when the polarization plane changes rapidly. There is a problem with followability. Furthermore, since there are three locations to be adjusted, there is a problem that the control system becomes complicated.

Method (2), which matches the polarization plane of the local LD to the polarization plane of the signal light at the output end of the optical fiber, which is the transmission path, takes a part of the signal light, which originally has low optical intensity, and divides it into two orthogonal waves using a wave plate, etc. After determining the intensity in the polarization direction, the polarization planes of the signal light and the local light are matched, and it requires large optical components such as an optical fiber type Faraday rotation element and a λ/4 rotation element, making it large in shape. It is something. Further, there were problems in that the level of received light was reduced and the precision in measuring the plane of polarization was low.

The essence of method (2) is to separate the received light into two orthogonal polarized lights using optical components such as a polarizing beam splitter or mode splitter, mix them with a local oscillating semiconductor laser, receive them separately, and combine them in the electrical domain. Generally speaking, two PDs, which are photodetectors, are required, and simply combining them into one will lower the detection efficiency, so it is necessary to match the phase of the electrical signals, which increases the size of the electrical circuit, etc. There was a problem. Furthermore, in the case of optical communication systems using wavelength division multiplex transmission, there are problems such as large demultiplexing loss.

On the other hand, there are two types of polarizing beam splitters that separate received light into two polarization direction components: bulk type and waveguide type. A Wollaston prism or the like is used as a bulk polarizing beam splitter, but this is large in shape and is not suitable for integration. Waveguide type polarizing beam splitters include circuit elements as described below.

FIG. 7 shows a stacked directional coupler type TE/TM mode splitter, that is, a polarizing beam splitter, in which two slab type single mode optical waveguides 71 and 72 are partially close to each other on a substrate 70.

This method takes advantage of the fact that the dispersion of TE and 7M modes in multilayer dielectric waveguides is significantly different.
The propagation constant of the guided mode of 2 is β1°β2, and the thickness of the low refractive index dielectric intermediate layer is set so that β1 and β2 are the same in TE mode, and so that β1 and β2 are significantly different in 7M mode. However, this was realized by making the length of the adjacent region, that is, the bonding region, the perfect bonding length in the TE mode. At this time, the TE mode of the light incident on the waveguide 71 is extracted from the waveguide 72, and the TM mode is extracted from the waveguide 72.
It will be taken out from 1.

However, in this type of TE/TM mode splitter, the thickness of the low refractive index dielectric intermediate layer must be set so that βl and β2 are the same in the TE mode, and the coupling must be made to have a perfect bond length. There is a problem in that it is very difficult to set the length of the area, and the performance varies widely. Furthermore, when used as an optical fiber sensor, since the waveguide basically has no lateral light confinement effect, optical components such as bulk lenses are required in addition to this mode splitter, and optical axis alignment work is required. It became.

In addition, three-dimensional optical waveguides 74, 75 as shown in FIG.
are placed close to each other in a part, and the length of the adjacent area is T.
Created to have a perfect bond length for E mode, and TE
A TE/TM mode splitter has been proposed in which a metal 76 having a negative dielectric constant is placed on top of a waveguide 75 through which a mode is output. This is because the effective refractive index of the waveguide 75 is reduced by about 10-'' in the 7M mode by placing the metal 76 with a negative permittivity on the waveguide 75, whereas in the TE mode it is reduced by about 1O'5. This takes advantage of the fact that the effect is one order of magnitude smaller.

This mode splitter also needs to realize a perfect coupling length, and the presence of a metal film causes problems such as increased loss of light in the 7M mode. Furthermore, there are also characteristic problems such as the mode splitting ratio being only about 1 OdB, and the problem that the characteristics deteriorate significantly even if the optical wavelength used changes by several nanoliters.

(Problems to be Solved by the Invention) As described above, the polarization plane control devices used in conventional optical fiber sensors and optical coherent communications are extremely difficult to manufacture, require a lot of manpower to adjust, and have a large shape. It had many problems, including a complicated control system.

In addition, conventional TE/TM mode splitters have the following problems: 1. The manufacturing method is very difficult and the performance varies widely. There were problems such as the need for axis alignment.

The present invention has been made in consideration of the above circumstances, and its purpose is to simplify the manufacturing method and adjustment process, increase reliability, and reduce the size of optical fiber sensors and optical fiber sensors. An object of the present invention is to provide a waveguide type polarization control device suitable for use in coherent communication.

Another object of the present invention is to provide a high-performance waveguide-type polarizing beam splitter that reduces variations in performance due to manufacturing methods, simplifies the alignment work process, increases reliability, and enables miniaturization. There is a particular thing.

[Structure of the Invention] (Means for Solving the Problems) The gist of the present invention is to separate the reception destination incident on a three-dimensional waveguide into two orthogonal polarization directions, and to set the polarization plane of one of the separated lights to 90°. The purpose is to match the planes of polarization to one by rotating the light by degrees and combining the light with the same phase with the other light.

That is, the present invention provides a waveguide-type deflection beam splitter that separates incident light into two orthogonal polarization direction components and propagates each component through a different optical waveguide, and a waveguide-type polarization beam splitter that separates the light separated by the beam splitter. Two 3s each propagating
dimensional optical waveguides, a TE/TM mode conversion element that rotates the polarization plane of guided light by 90 degrees on one of these optical waveguides,
A waveguide-type polarized waveguide comprising a phase shifter that changes the phase of waveguide-propagated light on at least one side of the optical waveguide, and an optical coupler that couples the light propagated in the optical waveguide, each integrated on the same substrate. The control device changes the coupler coupling amount and the shift amount of the phase shifter so that the output light intensity of the optical coupler becomes maximum.

The present invention also provides a waveguide-type polarizing beam splitter used in the above device, in which two three-dimensional single mode optical waveguides are integrally formed on a substrate, and at least one of these optical waveguides is bent, and the other optical waveguide is bent. Proximity regions are provided adjacent to the waveguide over a predetermined width at predetermined intervals so that 100% of the light emitted in the direction of the smaller electro-optic coefficient is coupled to the waveguide, and the electro-optic coefficient of the waveguide in the proximal portion is A plurality of electrodes that change the refractive index component in a large direction by an electro-optical effect are provided, and a dielectric layer having a refractive index lower than that of the waveguide core is provided between the electrodes and the waveguide, and a voltage applied to the electrodes is provided. By adjusting it, it is set so that light polarized in the direction with a large electro-optic coefficient is coupled at 0%.

(Function) According to the present invention, the optical fiber light that has entered the three-dimensional optical waveguide, that is, the received light, is separated into two orthogonal polarization bases by the waveguide type polarization beam splitter, and each is propagated to a different optical waveguide. Ru. One side of the light transmitted to the optical waveguide is TE
/The plane of polarization is rotated by 90 degrees by the TM mode conversion element,
Furthermore, the phases of the respective lights are aligned by a phase shifter. In this state, the two lights are combined by the optical coupler, so that the coupler provides received light with a controlled plane of polarization.

On the other hand, in a waveguide-type polarizing beam splitter, the - side optical waveguide couples 100% of the TE or TMM mode to the other optical waveguide, and 0% coupling of the other mode, so the received light is completely split into two. It becomes possible to separate the waves into two polarization directions.

(Example) Hereinafter, details of the present invention will be explained with reference to illustrated embodiments.

First, as shown in FIG. 1, an optical waveguide 11.
12, and further a waveguide type polarizing beam splitter 2
0. Consider a waveguide type optical integrated circuit formed with four elements: a TE-7M mode conversion element 30, a phase shifter 40, and an optical coupler 50. The front fiber 60 is connected to the entrance of the waveguide type polarizing beam splitter 20, and the two optical waveguides on the exit side of the polarizing beam splitter 20 are designated 11 and 12.

The signal light incident on the three-dimensional optical waveguide is separated into two polarized components, one in the direction perpendicular to the substrate and one in the horizontal direction, and propagated. These two polarization directions, namely TE.

The TM mode light is separated into optical waveguides 11 and 12 by a polarizing beam splitter 20.

Assuming that the mode of the light propagated to the optical waveguide 11 is now the TM mode, the propagated light of the optical waveguide 11 is converted to the TE mode by the TE-7M mode conversion element 30. Optical waveguide 12
The light propagating is in TE mode. Here, the phase of the light propagating in the optical waveguide 12 is changed by the phase shifter 40,
By matching the phase at the entrance of the optical coupler 50 with the light propagating through the optical waveguide 11, the light intensity output to the waveguide 11 on the exit side of the coupler 50 can be maximized.

Furthermore, since the polarization direction of the signal light at the output end of the optical fiber is not constant, the intensity of the light propagating through the optical waveguides 11 and 12 is not constant. Therefore, by changing the coupling ratio of the optical coupler 50 so that the light intensity at the exit becomes maximum, it is possible in principle to limit and control the polarization plane of the light emitted from the optical fiber to one direction.

Here, TE-TM mode conversion element 30 reconnaissance coupler 5
They may be arranged in any combination between 0 and 0. In addition, the optical waveguide 11 is
It is clear that it is not necessary that the TM mode light comes out of the optical waveguide 12 and the TE mode light comes out of the optical waveguide 12; the reverse is also possible.

Furthermore, it is clear that the phase shifter 40 need not be arranged in the optical waveguide 12, but may be arranged in the optical waveguide 11, or in both the optical waveguides 11, 12.

Next, a more specific example of the above device will be described.

First Example> In this example, as shown in FIG. 2, Li is used as a waveguide.
This is a case in which a three-dimensional optical waveguide in which Ti is thermally diffused on a NbO3 crystal substrate 10 is used. Polarizing beam splitter 2
0, the TE mode light is separated into the optical waveguide 11 and the TMM mode light is separated and propagated into the optical waveguide 12 using a Y branching element. TM-TE mode conversion element 30 in the optical waveguide 12
is arranged to convert TMM mode light into TE mode light.

Further, a phase shifter 40 having two electrodes and changing the phase by an electro-optic effect is arranged on the optical waveguide 11. Next, the optical waveguide 11 is connected to an optical coupler 50 having electrodes arranged on the waveguide of a Y-branch coupler.

12 lights are made incident. At the entrance of this coupler 50, TE
Only the mode destination is incident, and the optical waveguide 13 on the exit side
The voltage applied to the electrode above the coupler and the voltage applied to the phase shifter 40 are controlled so as to maximize the light intensity.

In the case of this embodiment, even if the polarization direction of the signal light from the end fiber changes, the exit optical waveguide 13 is connected to the TE/TM mode splitter 20. It has the characteristic that it is controlled by two TM-TE mode conversion elements 30 as light having only a TE mode component.

Furthermore, since it can be integrated on a single substrate, there is no need for adjustments such as alignment of optical axes, and it is easy to manufacture and there are no problems such as variations in performance. Furthermore, a waveguide type polarization plane control device that is much smaller than conventional devices, has an easy configuration of a control system, and has high stability and high performance can be obtained.

<Second Example> In this example, a three-dimensional optical waveguide in which Tj is thermally diffused onto a LiNbO3 crystal substrate is used as a waveguide. As shown in FIG. 3, a directional coupler branching element is used as a polarizing beam splitter 20 to separate and propagate TE mode light into the optical waveguide 11 and 7M mode light into the optical waveguide 12.

A TE-7M mode conversion element 30 is arranged in the optical waveguide 11 to convert TE mode light into 7M mode light.

Further, a phase shifter 40 having two electrodes and changing the phase by an electro-optic effect is disposed on the optical waveguide 12. Next, the propagating light of the optical waveguides 11 and 12 is made to enter an optical coupler 50 having an electrode arranged on the waveguide of the directional coupler. Only the 7M mode light is incident on the entrance of the coupler 50, and the voltage applied to the electrode of the coupler l' and the voltage applied to the phase shifter 40 are applied so as to maximize the light intensity of the optical waveguide 11 on the exit side. control.

In the case of this example, the polarization of the signal light from the optical fiber J°
Even if the direction changes, the exit optical waveguide 11 has a TE/T~1 mode splitter 20. It has the characteristic that it is controlled by two TE-7M mode conversion elements 30 as light having only a 7M mode component.

Also, the amount of coupling is determined so that the intensity of the signal light is maximized.

To adjust the amount of phase shift, it is sufficient to refer to the ratio of the light intensities from the optical waveguides 11 and 12, and it also has the feature that it has no effect even if the optical signal intensity on the transmitting side changes. moreover,
Since it can be integrated on a single substrate, there is no need for adjustments such as alignment of optical axes, and there are no problems such as variations in performance, which are easy to manufacture.

Furthermore, a waveguide type polarization plane control device that is much smaller than conventional devices, has an easy configuration of a control system, and has high stability and high performance can be obtained.

On the other hand, a structure as shown in FIG. 4 will be considered as a polarizing beam splitter used in the above-mentioned waveguide type polarization control device. In other words, two single-mode three-dimensional optical waveguides are close to each other, and the waveguide spacing G is small in the close area.
is constant, the width, that is, the length in the light propagation direction, is made to satisfy a predetermined length, and two electrodes having a distance G' smaller than the waveguide distance C are placed on top of the structure. Consider the body.

Here, the two electrodes 21.22 are connected to the optical waveguide 11.12.
The waveguide is placed on the surface of the waveguide via a thin insulator @23 which is a dielectric material with a lower refractive index. This proximity area is 1. Tokoro 311
In the directional coupler, the length and width G are set so as to provide a complete coupling length for a mode polarized in the direction of the smaller electro-optic coefficient, ie, either TE or TM mode. for example,
'If the electro-optic coefficient is large in the depth direction of the substrate,
When TE mode light enters the optical waveguide 11, the length and width G are set so that the TE mode light is coupled to the optical waveguide 12 and does not exit the optical waveguide 11 after passing through a nearby region. do. When light in both TE and 7M modes is input into this optical circuit, the 7M mode light will be emitted from both optical waveguides 11 and 12.

At this time, when a voltage is applied to the electrodes 21 and 22, a large electric field is applied in the depth direction of the substrate, and the refractive index in the depth direction of the substrate changes due to the electro-optic effect, causing propagation of only 7M mode light. The constant can be changed, and as a result, the optical coupling efficiency of 7M mode light to the optical waveguide 12 can be set to 0%.

Note that the insulator thin film 23, which is a dielectric material having a lower refractive index than the waveguide and is disposed between the electrode and the waveguide, is for reducing waveguide loss in the TM mode.

If the magnitude of the electro-optic coefficient is large in the direction perpendicular to the depth direction of the substrate, that is, if the design is designed to cause complete coupling in the 7M mode, it is possible to You can get the same functionality as with . At this time, the electrode spacing G' does not need to be smaller than the waveguide spacing G, and is approximately the same as the waveguide itself. In addition,
The width of the waveguide itself and the waveguide spacing G can generally be considered to be approximately the same.

In addition, it is clear that the position of the electrode and whether to set the perfect coupling length in TE or TM depends on the waveguide structure, that is, the magnitude of the electro-optic effect due to the crystal structure. By changing it, the characteristics can be improved. Furthermore, when the electro-optic coefficient is large in the depth direction of the substrate, the proximal region length is designed to be the perfect coupling length in TE mode, but L is designed close to the perfect coupling length in 7M mode. 1 for adjustment if
'G pressure decreases. Therefore, in order to drive at a low voltage, it is more desirable to design so that the perfect coupling length is obtained in the TE mode in the vicinity of the length where the perfect coupling length can be obtained in both modes. This is true even if the electro-optic coefficient is large in the direction perpendicular to the depth direction of the substrate.

Next, a more specific example of the waveguide type polarizing beam splitter will be described with reference to FIGS. 5 and 6. In this example, the optical waveguide substrate is LiNb0 with Ti thermally diffused.
This is the case when a 3Z plate is used, and Figure 5 shows the cross-sectional structure.
FIG. 6 shows the planar structure. The waveguide width is 8 μm, the waveguide spacing, that is, the spacing between adjacent regions, is 5 μm, and the coupling length, that is, the width of the adjacent region, is set to 9 mm.
This is a waveguide that underwent diffusion at 50°C for 5 hours.

Insulator 2 is a dielectric material with a lower refractive index than the waveguide.
3, a buffer layer of 5i02 was tried on the top of the 0.3 μm waveguide surface, and an electrode 21 of Cr/Au was placed on top of it.
21a, 21b), 22 (22a, 22b)
was tested with a spacing of 4 μm. Now, light with a wavelength of 1.3 μm is incident on the waveguide 11 in half of the TE mode and half of the light in the 7M mode. When no voltage is applied to the electrodes 21 and 22, the output side of the waveguide 12 has 100% output in the TE mode.
In M mode, 80% is emitted.

Here, when a voltage of about 7V is applied, since the waveguide is set directly below the electrode, the refractive index only in the Z direction, that is, the refractive index of 7M mode light, changes, and as a result, the coupling coefficient between the two waveguides changes. changes, almost no radiation is emitted to the waveguide 12 side, and 100% is emitted from the waveguide 11. The insertion loss at this time was about 0.8 dB or less, and a mode splitting ratio of about 20 dB was obtained.

It is clear that A/2O may be used as the buffer layer 23, and as shown in FIG. 5, long-term stability is obtained by cutting at intervals equal to or smaller than the electrode interval. It is a good idea for this purpose.

The structure of the electrodes 21 and 22 is Ti/Au.

T i / P t / A u etc. may be used, and I
It may be t', a simple metal, or a metal compound such as AuSi. Furthermore, even if the coupling length is not 100% in the TE mode due to the fabrication method during waveguide patterning, the 7M mode light will not be emitted to the waveguide 12.

In this waveguide-type polarizing beam splitter, even if the used optical wavelength changes by several nanoseconds, the mode separation ratio on the waveguide side where one polarized wave is emitted may decrease slightly, but the voltage on the remaining waveguide side may decrease slightly. The characteristics will not deteriorate by adjusting again.

[Effects of the Invention] As described above, according to the present invention, a waveguide-type polarizing beam splitter is provided on a substrate that includes an optical waveguide.

Since the structure includes a phase shifter and an optical coupler for the mode conversion element 1, it reduces performance variations due to manufacturing methods, simplifies the axis alignment process, achieves high reliability and miniaturization, and allows the configuration of the control system to be A waveguide type polarization control device that is easy, highly stable, and has high performance can be realized. Therefore, it is extremely suitable for use in optical fiber communications and optical coherent communications.

[Brief explanation of the drawing]

1 to 6 are for explaining one embodiment of the present invention. FIG. 1 is a plan view showing the basic configuration of a waveguide type polarization control device, and FIGS. 2 and 3 are the same. Figure 4 is a plan view showing a specific example of the device, Figure 4 is a perspective view showing the basic configuration of a waveguide type polarizing beam splitter, Figure 5 is a sectional view showing the specific configuration of the beam splitter, Figure 6 is a perspective view of the same beam A plan view showing a specific configuration of the splitter, and FIGS. 7 and 8 are diagrams each showing a schematic configuration of a conventional polarizing beam splitter. 10... Crystal substrate, 11, 12.13... Optical waveguide, 20... Polarizing beam splitter, 21.22...
Electrode, 30... TE/TM mode conversion element, 40...
- Phase shifter, 50... optical coupler, 60... optical fiber. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

Claims (2)

[Claims] (1) Separate the incident light into two orthogonal polarization direction components,
A waveguide-type polarizing beam splitter that propagates different optical waveguides for each component, two three-dimensional optical waveguides that respectively propagate the light separated by this polarizing beam splitter, and one of these optical waveguides. a TE/TM mode conversion element that rotates the polarization plane of the waveguide-transmitted light by 90 degrees; a phase shifter that changes the phase of the waveguide-propagated light on at least one of the optical waveguides;
optical couplers that couple two waveguide propagating lights, respectively, are integrated on the same substrate, and the coupler coupling amount and the shift amount of the phase shifter are changed so that the output light intensity of the optical coupler is maximized. A waveguide type polarization plane control device characterized by the following. (2) Two three-dimensional single mode optical waveguides that are integrally formed on a substrate and have proximate regions that are adjacent to each other over a predetermined width at a predetermined interval, and in the proximal regions of these optical waveguides. A plurality of electrodes are provided via a dielectric layer having a refractive index lower than that of the waveguide core portion, and are configured to change the refractive index component in a direction in which the electro-optic coefficient of the optical waveguide in the adjacent region portion is large by an electro-optic effect. The spacing and width of the adjacent areas of the optical waveguides are set such that approximately 100% of light polarized in a direction with a smaller electro-optic coefficient is coupled to one optical waveguide with respect to the other optical waveguide, and the electrode A waveguide type polarizing beam splitter, characterized in that a voltage applied to the waveguide type polarizing beam splitter is set so that light polarized in a direction with a large electro-optic coefficient is coupled at approximately 0%.