JPS63147145A - Waveguide type mach-zehnder interferometer - Google Patents

Abstract

PURPOSE:To attain stable interferometer operation which does not depend upon the polarization direction of incident light by controlling the birefringent values of two single-mode optical waveguides locally through the operation of an applied adjustment groove. CONSTITUTION:When the difference in effective refractive index between a TM wave which has a polarization direction at right angles to a substrate and a TE wave which has a polarization direction in parallel to the substrate is a birefringent value B, the difference R in optical path length between optical waveguides 4 and 5 which depends upon the polarization directions is the difference between line integral values integral Bdl1 and integral Bdl2 of the value B along the respective optical waveguides, where l1 and l2 are line coordinates along the two optical waveguides 4 and 5. Applied adjustment grooves are provided on both sides of at least one optical waveguides along the optical waveguide so that the R is an integral multiple (including 0) of the wavelength of in-use light. Consequently. The Mach-Zender light interferometer performs the same operation with an optical frequency multiplexer demultiplexer without depending upon the polarization state of the incident wave.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a waveguide Mach-Zehnder optical interference S1 that is capable of stable operation regardless of the polarization direction of incident light.

[Prior art and problems]

Optical interferometers that connect two optical couplers, such as directional couplers, with two optical waveguides (M) are known as Mach-Zehnder optical interferometers, switches, optical sensors, and more recently, frequency It is used in multiplex optical communication multiplexer/demultiplexer, etc. This Mach-Zehnder optical interferometer has a (1
) Bulk type, (2) fiber type, and (3) waveguide type, but the waveguide type constructed on a flat substrate is considered the most promising for reasons such as reliability, productivity, small size and light weight. has been done.

In addition, the Mach-Zehnder optical interferometer has the following characteristics in terms of optical path configuration:
It can also be classified into (a) symmetrical type and (b) asymmetrical type. A symmetric type is one in which the lengths of the two optical waveguides connecting two optical couplers are equal, and an asymmetric type is one in which the lengths are intentionally made different.

FIG. 3 is an explanatory diagram of the configuration of a conventional asymmetric waveguide optical interferometer configured for application to an optical frequency multiplexing/demultiplexing device, where (a) is a plan view and (b) is an (aJ Line segment Δ in the diagram
It is an enlarged cross-sectional view along A'.

In the figure, a directional coupler 2.3 formed of a silica-based glass material on a silicon substrate 1 consists of two adjacent silica-based single-mode optical waveguides, each of which has a coupling rate of approximately 50%. It is set to be. Further, the two optical waveguides 4.5 connecting between the directional couplers 2.3 have a length as short as Δ.

In such a Mach-Zehnder type optical interferometer, when the optical frequency of the signal light incident from the input boat 1a is changed, 8f=□・□ 2n ΔL (C is the speed of light, n is the refractive index of the optical waveguide) ) It is known that signal light can be taken out alternately to the output ports 1b and 2b with a period of . Therefore, for example, 1
．． In the 55 μm band, two signal lights fl separated by an optical frequency interval of Δf=10GI12.

When f2 is input simultaneously from input capo 1-1a, if ΔL is set to 10 mm according to the above formula, two signal beams f, f2 can be separated and taken out to output boats 1b and 2b. . In reality, the first period of the Mach-Zehnder optical interferometer is the signal light f+.

In order to synchronize with the frequency value of f2 and take out the desired signal light to the desired output port, the effective optical path length of the optical waveguide 5 is set to 1 by the thermo-optic effect at the top of one of the optical waveguides 5. A thin film heater 6 is used as a phase shifter to change the wavelength.
The optical interferometer shown in FIG. 3 functions as an optical frequency multiplexer/demultiplexer as a whole.

However, this conventional waveguided optical interference method 1 has the following problems. In other words, since the thermal expansion coefficients of the silicon substrate 1 and the optical waveguide 4.5 formed thereon are different, the optical waveguide is subjected to compressive stress in a direction parallel to the substrate, which causes stress birefringence. Therefore, the effective refractive index n
differs slightly depending on the polarization direction of the incident light. Therefore, there is a problem that unless the polarization direction of the desired light is set to one of the two, the operation as an optical frequency multi-Φ multiplexer/demultiplexer becomes completely impossible.

The present invention solves the drawbacks of conventional waveguide optical interferometers and provides a waveguide Mach-Zehnder optical interferometer that does not depend on the polarization direction of incident light.

[Means for solving problems]

One possible solution to the problems of the prior art is to reduce the birefringence of the optical waveguide to zero, but what about optical waveguides formed on a flat substrate? It is extremely difficult in manufacturing to achieve zero ll1li'I fold.

In contrast, the present invention recognizes the existence of birefringence in the optical waveguide and effectively solves the polarization dependence of the Mach-Zehnder optical interferometer.

If the difference in effective refractive index between a TM wave with a polarization direction perpendicular to the substrate and a TE wave with a polarization direction parallel to the substrate is defined as the birefringence value B, then the optical waveguide 4.5 depending on the polarization direction (Fig. 3) ) is given by the following equation.

R= f 8dJ + −f BdJ 2
...(1) Here, J+ and J2 are the line coordinates along the two optical wave paths 4.5, respectively. Also, f B
dJ + and fBdJ2 are line integral values of the B value along each optical waveguide, and the integral range is from directional coupler 2 to directional coupler 3.

The main feature of the present invention is that the B value is locally adjusted so that R is an integral multiple (including O) of the wavelength of the light used. In other words, since an optical phase difference that is an integral multiple of the optical wavelength λ cannot be identified by a Mach-Zehnder optical interferometer, it appears that the interference conditions for the TM wave match those for the T and E waves. The focus is on Specifically, the local adjustment of the B value is achieved by providing applied adjustment grooves along at least one of the optical waveguides on both sides of the optical waveguide.

With these settings, the discrepancy between the TM wave separation conditions and the TE wave separation conditions, which was a conventional problem, is resolved, and the Mach-Zehnder optical interference π1 is eliminated under the same film heater phase shifter driving conditions.
It becomes possible to perform the same operation as an optical frequency multiplexing/demultiplexing device regardless of the polarization state of the required wave.

Hereinafter, the present invention will be explained in detail with reference to Examples.

Elements indicated by the same reference numerals rNll in the figures shown below represent the same thing.

[Embodiment 1] FIG. 1 is a diagram illustrating a first embodiment of the present invention, in which (a) is a plan view of a waveguide Mach-Zehnder optical interferometer;
(b) is an enlarged cross-sectional view taken along line segment AA' in figure (a). The Mach-Zehnder optical interferometer of the embodiment shown in this figure and the conventional Mach-Zehnder optical interferometer shown in FIG. stress adjustment groove 21a for locally changing stress;
The difference is that 21b is formed.

In this example, a 50 μm thick silica glass cladding H12 is formed on a 0.7 mm thick silicon substrate 1, and two silica glass cores are formed in each cladding layer 12. portion constitutes two optical waveguides 4.5. These two silica-based glass optical waveguides 4 and 5 are adjacent to each other so as to be evanescently coupled to form a directional coupler 2.3 with a coupling ratio of 50%.

The cross-sectional dimensions of the optical waveguide 4.5 are set to approximately 6μ7rL×6μm, and the relative refractive index difference with the cladding layer 12 is 0.7.
It is 5%. Further, the curved portions of the optical waveguides 4 and 5 are configured to have a radius of curvature of about 5 mm. Such a silica-based single mode optical waveguide has Si C1a, Tt C
It can be produced by a well-known method combining a glass film deposition technique using a flame hydrolysis reaction of raw material gas such as No. 14 and a reactive ion etching technique. Stress adjustment groove 218.2
1b is formed by removing a portion of the cladding layer 12 on both sides of the core portion of the optical waveguide 4 by reactive ion etching. Therefore, the stress adjustment grooves 21a and 21b formed on both sides of the waveguide 4 function to relieve the compressive stress in the waveguide width direction that the waveguide 4 receives from the GT plate 1. Optical waveguide 4 in stress adjustment groove 2La, 21b forming region
Assuming that the quality of J is J12, the optical path length difference R between the two optical waveguides, which depends on the polarization direction given by the above equation (1), is expressed by the following equation.

R=8・ΔL−(B−8”) J 12・
... In formula (2), ΔL is the difference in length between the two optical waveguides, and in this example, ΔL was set to 10 m. Further, B is the birefringence value of the optical waveguide 4 in the region where the stress adjustment grooves 21a and 21b are not formed, and in this example, B≠4×10 −4. Further, B9 is the birefringence value of the optical waveguide 4 in the region where the stress adjustment grooves 21a and 21b are formed. B9 is the stress adjustment groove 21a, 21
The width W of the cladding FA sandwiched by b (Fig. 1 (b
, l), here ~V#150μ
By choosing m, the birefringence value is reduced by half, and B=
2x10-' (in general, 89 decreases as W decreases).If the birefringence structure of the optical interferometer is designed so that R is an integral multiple of the wavelength λ as described above, the polarization dependence of the input tA can be reduced. However, in this example, B≠4×10−4 cars, B #2×10−′, △L 10m=10′ 1
If Jt2# is set to 12.3m corresponding to 1m, R# can be adjusted to 1.55μm, that is, 1 times the wavelength of the light used, from the above equation (2).

In fact, it has been shown that the Mach-Zehnder optical interferometer configured with the above numerical example exhibits stable operation as a multiplexer/demultiplexer for optical frequency multiplexing communications, regardless of the polarization direction of the incident signal light.
Approved.

It should be noted that the present invention is not limited to the above-mentioned combination of B* and J12, and there are of course various combinations within the range that satisfies formula (1) or formula (2). For example, if W is set to about 90μ7n, B"=
It becomes ixio-4, but in this case J+? It is also possible to eliminate light leakage dependence by setting #13.3m and adjusting R#O, that is, 1≧0 times the used light wavelength.

[Example 2] FIG. 2 is an explanatory diagram showing a second example of the present invention. Similar to Example 1, a directional coupler is formed using a silica-based single mode optical waveguide 4.5 on a silicon substrate 1. An asymmetrical Mach-Zehnder optical interferometer that connects 2゜3 (to optical path length difference L #5
mm) is constructed. Contrary to Embodiment 1, a stress adjustment groove 21a is provided along a part of the shorter optical waveguide 5 over a length of 121.

21b is formed. In this case, R given by the above formula (1) is expressed as follows.

R=[3・Δl+ (B-8”)J2+...(
3) Therefore, by setting ΔL #5M, B #4
By adjusting R twice the wavelength of 1.55 μm, Mach
We were able to effectively eliminate the polarization dependence of the Zehnder optical interferometer.

In addition, Example 1. In either case of the second embodiment, the thin film heater phase shifter 6 changes the optical path length difference between the two optical waveguides by about one wavelength in accordance with the optical frequency value of the signal light, and changes the frequency of the Mach-Zehnder optical interferometer. Since the purpose is to tune the separation period to the frequency value of the signal light, it may be provided on the optical waveguide 4 instead of on the optical waveguide 5.

Further, the thin film heater phase shifter 6 is based on the thermo-optic effect principle, and its phase shifting action is isodirectional, that is, TE wave, T
Since it works equally well on both M waves, the polarization dependence is
Please note that there is no concern that this will occur.

Furthermore, in the above embodiment, a directional coupler was used as the optical coupler 1 for optical interference, but instead of the directional coupler, a Mach-Zehnder optical interferometer was used using a Y-shaped branch/combiner. Those configured are also included within the scope of the present invention.

Further, in the above embodiment, the depth of the stress adjustment itA is set to the entire thickness of the cladding layer, but this depth may be an intermediate value, and B* decreases as the depth of the groove increases.

〔Effect of the invention〕

As explained above, in the present invention, the birefringence value of the two A-mode optical waveguides constituting the asymmetric waveguide Mach-Zehnder optical interferometer is locally controlled over a specific length by the action of the applied adjustment groove. This effectively eliminates the polarization dependence of the optical interferometer, and has the advantage of realizing stable optical interferometer operation that is independent of the polarization direction of the incident light. That is, it is possible to provide an optical frequency multi-single circuit, an interferometer type optical sensor circuit, etc. without using an unnecessary optical device such as a polarization controller.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a 7-Zehnder optical interferometer according to a first embodiment of the present invention, in which (a) is a plan view, and (b) is an enlarged cross-sectional view at line segment A△' in figure (a). , FIG. 2 shows the second embodiment of the present invention.
A configuration plan view of Mach-Zehnder optical interference C1 of an actual flow example,
FIG. 3 is a block diagram of a conventional Mach-Zehnder optical interferometer. 1... Board, la, 2a-manpower port, 1b, 2b・
... Output port, 2.3... Directional coupler, 4.5.
...Optical waveguide, 6... Thin film heater phase shifter, 21a, 2
1b... Stress adjustment groove. Figure 1 Figure 2 Figure 3 (b)

Claims (2)

[Claims] (1) In a Mach-Zehnder optical interferometer in which two optical couplers are connected by two stress-birefringent optical waveguides of different lengths, the waveguide birefringence B of each optical waveguide is Stress adjustment grooves are provided along at least one of the optical waveguides on both sides of the optical waveguide so that the difference in line integral values between the two optical couplers is approximately equal to an integral multiple of the used optical wavelength λ. A waveguide Mach-Zehnder optical interferometer. (2) The waveguide Mach-Zehnder optical interferometer according to claim 1, wherein the optical waveguide is a silica glass single mode optical waveguide formed on a silicon substrate.