JPS63182608A - Waveguide type polarized light separating element - Google Patents

Abstract

PURPOSE:To permit formation of a waveguide type polarized light separating element on a plane substrate by adjusting the double refractive index value distribution of light guides in such a manner that the difference of the curvilinear integrals of the double refractive index values along the two light guides is effectively half the wavelength of the used light. CONSTITUTION:The adjustment is so made by the effect of the stress relieving grooves 7a, 7b provided at the mid-point of the light guides 4, 5 and a thin film heater 8 that the effective optical path lengths of the light guides 4, 5 are equalized with respect to the signal light (TE wave) having an electric field component parallel with the substrate 1 and the difference in the effective path lengths is lambda/2 with respect to signal light (TM wave) having an electric field component perpendicular to the substrate 1. While the TE wave component of the incident signal light on an input port 11 is emitted only to an output port 12a, the TM wave component is emitted only to an output port 11a and functions as the waveguide type polarized light separating element. Formation of the waveguide type polarized light separating element on the plane substrate is thus permitted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a waveguide polarization separation element used for polarization plane separation of optical signals in the fields of optical fiber communications, optical fiber sensors, and the like.

(Conventional technology and its problems) With the rapid progress of optical industry technology such as optical fiber communication,
Demand for various optical circuit components is increasing. Optical circuit components are classified into (1) bulk type.

It can be classified into three types: (2) fiber type, and (3) waveguide type. However, for reasons such as reliability, production time, small size and light capacity, and possibility of integrating functions, the fiber type is constructed on a flat substrate. Waveforms are considered the most promising.

A waveguide type optical circuit component is constructed using a guiding waveguide formed on a flat cover plate as a basic element.

Among them, silica-based optical waveguides that can be fabricated on silicon substrates have a core section with a cross-sectional dimension of about 5 to 10 μm for single mode and r: Since it can be set to about 50 μm, it is expected to be a means for realizing practical waveguide optical circuit components with excellent compatibility with optical fibers. (Reference: Masao Kawachi, “Microfabrication of silica-based optical waveguides,” Journal of the Micro-Optics Research Group, Optics Conference, Japan Society of Applied Physics, 1986.4
/vol, NO, 2, 1) 0.33-38) By the way, in the fields of single-mode optical fiber communications, especially coherent optical communications and single-mode optical fiber sensing, control of the plane of polarization of signal light is an important issue. It has become a matter of interest. A polarization splitting element can be cited as a basic optical circuit component for polarization plane control, but until now, effective polarization splitting element configuration means have not been known in the technical field of silica-based single mode optical waveguides. Despite having many advantages for this reason, waveguide optical circuit components using silica-based single mode optical waveguides have a polarization separation effect that cannot be achieved with bulk type optical circuit components such as externally installed Glan-Thompson prisms. This was a major obstacle in providing practical optical circuit components.

In view of the above circumstances, the present invention aims to realize a waveguide optical circuit component on a flat substrate using a birefringent single mode optical waveguide, and to provide a practical waveguide optical circuit component. There is.

[Means for solving problems]

The present invention has a basic configuration of a Matsuha-Zehnder type optical interferometer in which two directional couplers are connected on a substrate by two birefringent single mode optical waveguides of approximately equal length, and The birefringence value distribution of the optical waveguide is adjusted so that the difference in the line integral of the birefringence value along the optical waveguide is effectively 1/2 of the wavelength of the used light. It is something.

The birefringent single-mode optical waveguide described above is formed as a quartz-based single-mode optical waveguide with a core part buried in a cladding layer on a substrate, and a stress release groove is formed along one of the optical waveguides. It is desirable to adjust the stress birefringence value distribution, and it is also desirable to have a configuration in which a thin film heater is loaded on the top of one of the optical waveguides to finely adjust the optical path length.

[Effect]

The present invention combines the birefringence of two birefringent single-mode optical waveguides formed on a planar substrate and the optical phase sensitivity of a Matsuha-Zehnder type optical interferometer composed of these optical waveguides. This achieves the polarization separation effect.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to FIGS. 1 to 4.

FIG. 1 is a diagram illustrating the configuration of an embodiment of the present invention, in which (a) is a plan view, and (b) is a line segment AA' in (a).
FIG. In the figure, reference numeral 1 is a silicon substrate, 2 and 3 are directional couplers (3dB couplers) with a coupling rate of 50%, and 4.5 is a directional coupler 2°3 that is combined to form a Matsuha-Zehnder type optical interferometer. 6 is a silica-based glass cladding layer, 7a and 7b are stress release grooves as stress birefringence control parts, 8 is a thin film heater as a phase controller, 11.12 is an input port , 11a.

12a is an output port.

The optical waveguides 4 and 5 described above are formed by embedding a core part in a cladding layer 6, and the cross-sectional dimension of the core part is about 10 μm, and the thickness of the cladding layer 6 is 50 μTrL8.
! The silicon substrate 1 has a thickness of 0.4 to 1
It is about #.

The directional couplers 2 and 3 connect the two optical waveguides 4.5 by several μm.
The left end of the directional coupler 2 and the input port 11 are arranged parallel to each other over a length of several inches.
, 12, and the right end of the directional coupler 3 and the output port 11
a and 12a are connected by the above-mentioned optical waveguides 4 and 5, respectively.

The signal light incident from the input port 11 is sent to the directional coupler 2
The light is divided into two equal parts and distributed to the optical waveguides 4 and 5 by this action. The optical waveguides M4 and 5, which are connected to the directional coupler 2.3 to form an optical interferometer configuration, have the same structure and have an optical path length L+,
12 are equal, the signal light that has propagated through the optical waveguides 4 and 5 is directed to the output port 12 by the action of the directional coupler 3.
It is known that the radiation is emitted at a. Also, Ll! −
It is known that when the wavelengths λ and 2 differ by half the wavelength λ of the signal light, that is, λ/2, the signal light is conversely emitted to the output port 11a.

Therefore, in this embodiment, the stress release grooves 7a and 7b provided in the middle of the optical waveguides 4 and 5 and the action of the thin film heater 8 prevent the signal light (TE wave) having an electric field component parallel to the substrate 1 from being guided. The effective optical path lengths of the wave paths 4 and 5 are adjusted to be equal, and for signal light (TM wave) having an electric field component perpendicular to the substrate 1, the effective optical path length difference is adjusted to λ/2. ing. Under these adjusted conditions, the input boat 11
The TE wave component of the input signal light is output only to the output port 12a, whereas the TM wave component is output to the output port 11a.
This embodiment functions as a waveguide polarization splitting element. That is, in this example, by combining the birefringence of the silica-based single mode leading waveguide and the phase sensitivity of the Matsuha-Zehnder type optical interferometer,
The desired polarization separation effect has been achieved.

Next, a more specific configuration of the waveguide polarization splitting element of this example will be explained in more detail while explaining the manufacturing process using No. 21M (a) to (e).

First, as shown in (a), Sj
The lower cladding layer 21 and the core layer 22 are sequentially deposited using CR4 or TL C9a as a starting material gas by a known glass film deposition method using a flame hydrolysis reaction. That is, the lower cladding layer 21 is composed of 5j02, and the core layer 2
2 is composed of 5102 to which a small amount of TL02 is added as a dopant for controlling the refractive index. Next, as shown in (11), unnecessary portions of the core layer 22 are removed by reactive ion etching to form ridge-shaped core portions that constitute the optical waveguides 4 and 5, and then in (C). As shown, the upper cladding layer 23 is deposited again to cover the core portions by the glass film deposition method using flame hydrolysis reaction,
Together with the lower cladding layer 21, the cladding layer 6 is formed. As a result, optical waveguides 4 and 5 are formed.

In the state shown in FIG. 2(C), a strong horizontal compressive stress acts on the core portions of the optical waveguides 4 and 5 due to the difference in thermal expansion coefficient between the silicon substrate 1 and the silica glass.
It exhibits a certain degree of stress birefringence. Here, Bo is TM
It is the difference between the effective refractive index perceived by waves and the effective refractive index perceived by TE waves. However, in state (C), the optical path lengths of the two optical waveguides 4 and 5 that connect the directional couplers 2 and 3 are set equal, so despite the birefringence of the optical waveguides 4 and 5, First, there is no polarization direction dependence in the optical path length difference between the optical waveguides 4 and 5, and therefore no polarization separation effect occurs in this state.

Therefore, as shown in (d), stress release grooves 7a are provided on both sides of the core portion of one optical waveguide 4 as a stress birefringence control portion to partially release the compressive stress from the substrate 1.

7b is formed along the core portion by a length J as shown in FIG. 1(a).

FIG. 3 shows the width W of the ridge-shaped cladding layer 6 defined by the stress release grooves 7a and 7b and the normalized birefringence value B.
/ This shows the relationship with B o. Figure 3 shows
It was calculated based on stress distribution analysis results using the finite element method, and good agreement with experimental results was obtained. As shown in FIG. 3, for example, when W# is 200 μm, B/Bo# is 0°5, so the birefringence change ΔB due to the formation of the stress release grooves 7a and 7b is ΔB ', and the length J of the stress release grooves 7a and 7b is Δ
B・J−λ/2, that is, the optical wavelength used is λ−
In the case of 1.3 μm, if it is set to J-3゜3 mm, it becomes possible to give the optical path length difference of the optical waveguide 4゜5 a change of λ/2 that depends on the polarization direction. .

Next, as shown in FIG. 2(e), while maintaining the polarization dependence of λ/2, the optical path length is finely adjusted isotropically (without polarization dependence) to direct the polarized light to the output ports 11a and 12a. A thin film heater 8 is installed on the optical waveguide 5 to adjust the separation property of the polarized light beam splitter, and the manufacturing process of the polarization splitting element is completed. The thin film heater 8 is a nichrome metal film deposited over a width of about 50 μm and a length of about 5 mm, and is used to finely adjust the optical path length of the optical waveguide 5 isotropically by a so-called thermo-optic effect.

Note that this thin film heater 8 can be omitted, but generally, the optical path length of the optical waveguide often changes by the wavelength order by going through the manufacturing process shown in FIG.
It is desirable to install a membrane heater 8 in advance.

Figure 4 shows the polarization separation characteristics of the polarization separation element manufactured by the above steps and setting conditions.
The ratio of the TM acid component to the TE acid component of the signal light emitted from a and 12a is shown as a function of the power consumption PH of the thin film heater 8. The input port 11 receives circularly polarized light having a 1:1 ratio of TM acid component to TE acid component. As shown in FIG. 4, at PH=0.2 Watts, the output port 11
Only the TE wave is emitted to output port 12a1.a. :TM
A phase-matched state in which only waves are emitted is achieved, and a good function as a polarization separation element is obtained.

A good polarization separation function is also obtained at PH=0.7 Watts, but the output ports for the TE wave and TM wave are reversed from those in the above case. This corresponds to the fact that the optical path length changes by λ/2 between PH=0.2 Watt and PII=0.7 Watt.

In this way, the thin film heater 8 can also function as a kind of switch regarding polarization separation by utilizing the thermo-optic effect.

In addition, in the above embodiment, it was set so that ΔB-J=λ/2, but in general, ΔB-J-(N+l/2
)λ (N is an integer), a similar polarization separation effect can be obtained. However, if N is set too large (INI>5), the asymmetry of the Matsuha-Zenda type optical interferometer becomes strong.
This is undesirable because the wavelength dependence characteristic of the asymmetric Matsuhaka-Zehnder type optical interferometer becomes strong, and the wavelength band in which it can operate as a polarization separation element becomes narrow.

Further, in the above embodiment, the stress release grooves 7a and 7b were provided in the optical waveguide 4 as stress birefringence control parts, but in a more general description, the line integral value of the birefringence value along the optical waveguide 4 (integration range is between two directional couplers) and optical waveguide 5
Since it is sufficient that the difference in the line integral values along the line is effectively λ/2, the positions, shapes, etc. of the stress release grooves 7a and 7b can be selected arbitrarily without any problem. For example, stress release grooves 7a, 7
Of course, there is no problem even if b is provided on the optical waveguide 5 side.

In addition to the stress release grooves 78 and 7b used in the above embodiments, the stress birefringence control section may be formed by loading an appropriate material (stress applying section) near the core of the optical waveguide, or by using an external force. It is also possible to adopt a method such as applying compressive stress locally.

Further, in the above embodiment, the thin film heater 8 is connected to the optical waveguide 5.
However, even if the thin film heater 8 is moved to the top of the optical waveguide 4, the fine adjustment of the optical path length difference is relative, so a polarization separation effect can be obtained under appropriate heater driving conditions. .

Furthermore, in the above embodiment, a thin film heater 8 is used as a phase controller.
was used, but the present invention is not limited to this,
For example, a method may be adopted in which a narrow gap is provided in a part of the optical waveguide and the gap is trimmed by an etching technique to meet a predetermined phase matching condition.

Further, the present invention is not limited to a quartz-based single-mode optical waveguide on a silicon substrate, but can be similarly applied to single-mode optical waveguides made of other materials.

〔Effect of the invention〕

As described above in detail, according to the present invention, the difference in line integrals of birefringence values along the two optical waveguides is adjusted so that it is effectively 1/2 of the wavelength of the used light. A waveguide polarization separation element can be realized on a flat substrate using a birefringent single mode optical waveguide, making various optical circuit components for polarization control in the fields of optical communications and optical fiber sensors small, lightweight, and inexpensive. It has great advantages in providing Another advantage is that multifunctional optical waveguide circuits and optical integrated circuits can be constructed by taking advantage of the fact that it can be integrated on a flat substrate.

[Brief explanation of the drawing]

1 to 4 are diagrams for explaining one embodiment of the present invention. FIG. 1 is an explanatory diagram of the configuration of this embodiment, in which (a) is a plan view, (b) is an enlarged sectional view, and the second
Figures (a) to (e) are diagrams for explaining the manufacturing process of this example in order of process, Figure 3 is a diagram of predicted birefringence value by finite element method, and Figure 4 is a diagram explaining the operation of this example. It is. 1... Silicon substrate, 2, 3... Directional coupler, 4
゜5... Optical waveguide, 6... Clad layer, 7a, 7b
... Stress release groove, 8... Thin film heater.

Claims (1)

[Claims] 1. Basic configuration of a Matsuha-Zehnder type optical interferometer in which two directional couplers are connected on a substrate by two birefringent single-mode optical waveguides of approximately equal length. and the birefringence value distribution of the optical waveguide is adjusted so that the difference in the line integral of the birefringence value along the two optical waveguides is effectively 1/2 of the wavelength of the used light. A waveguide polarization separation element characterized by: 2. The two birefringent single-mode optical waveguides are silica-based single-mode optical waveguides whose core portions are buried in a cladding layer on the substrate, and the two birefringent single-mode optical waveguides are quartz-based single-mode optical waveguides whose core portions are buried in a cladding layer on the substrate. A stress release groove is formed in a part of the cladding layer, and the stress birefringence value distribution of the optical waveguide caused by the difference in thermal expansion coefficient between the substrate and the optical waveguide is adjusted. The waveguide polarization splitting element according to item 1. 3. The waveguide according to claim 2, wherein at least one of the two birefringent single mode optical waveguides is further loaded with a thin film heater for finely adjusting the optical path length. Polarization separation element.