JPH08334639A - Mach-zehnder optical circuit - Google Patents

Abstract

PURPOSE: To provide a Mach-Zehnder optical circuit with which the stable execution of wavelength multiplexing and demultiplexing to temp. is possible and there is no need for heating with heaters. CONSTITUTION: The Mach-Zehnder optical circuit of a waveguide type is constituted by connecting the phase shift part 14b consisting two pieces of waveguides 15, 16 which are formed on a substrate 21 and the composed of signal mode channel waveguides consisting of cores 23a having a rectangular shape in section and the structure obtained by embedding the circumferences of these cores 23a with clads 24 having the refractive index smaller than the refractive index of the cores 23a and vary in the length from two directional couplers 13a, 13b consisting of two pieces of parallel channel waveguides between these directional couplers 13a, 13b. Negative refractive index material 25 consisting of materials having the negative coefft. of temp. of the refractive index are disposed on the waveguides at least the phase shift part 14b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical component, and more particularly to a Mach-Zehnder optical circuit.

[0002]

2. Description of the Related Art Wavelength Division Mu
Lti / demultiplexing (WDM) is a promising future communication system because it can significantly increase the optical communication capacity and build a flexible system. In order to further increase the capacity, a narrow band WDM (Frequency Divisio) that multiplexes wavelengths with narrow channel spacing is used.
nMulti / demultiplexing)) is attracting attention. This system requires an optical multiplexer / demultiplexer that multiplexes and demultiplexes light of two different wavelengths (frequency), and a Mach-Zehnder optical circuit using a silica glass waveguide is considered promising.

FIG. 5 is a plan view of a conventional Mach-Zehnder optical circuit, and FIG. 6 is a sectional view taken along the line AA.

As shown in FIGS. 5 and 6, a buffer layer 22 is formed on a substrate 21, and two input waveguides 11a and 11b composed of cores 23a and 23b and a clad 24 are formed on the buffer layer 22. Are formed. Both input waveguides 11a and 11b are respectively connected to the input ends of the input side directional coupler (3 dB directional coupler) 13a, and the output ends of the input side directional coupler 13a differ in length by ΔL. Two waveguides (hereinafter referred to as "arms") 15,
16 are connected.

A heater 17 is provided on one arm 15 (upper side in the drawing). The arms 15 and 16 and the heater 17 form a phase shifter 14a.
Both arms 15 and 16 are connected to the input end of the output side directional coupler (3 dB directional coupler) 13b, and the output end of the output side directional coupler 13b is two output waveguides 12a and 12b.
Respectively connected to.

In such a Mach-Zehnder optical circuit, the wavelengths λ ₁ and λ ₂ are input to the port 1 which is one of the input ports.
Consider the case where light is input.

Light is radiated by the directional coupler 13a on the input side.
It branches into 1: 1 and propagates through the arms 15 and 16, respectively. Since the arm 15 is longer than the arm 16 by ΔL, a phase shift occurs. When the lights propagating through both arms 15 and 16 are again coupled to the output side directional coupler 13b, light of wavelength λ ₁ is output from port 3 and light of wavelength λ ₂ is output from port 4 due to optical interference. The demultiplexing condition at this time is Equation 1
Given by the formula.

[0008]

[Formula 1] Δλ = λ ₁ −λ ₂ = λ ₁ · λ ₂ / (2 · n _eff
ΔL) (where n _{eff is} the equivalent refractive index of the arm, ΔL is the waveguide length difference between both arms) Normally, in a narrow band WDM, Δλ is 0.01 nm to
It is 1 nm.

N _eff is about 1.47 (in the case of an arm made of quartz glass) When the wavelengths λ ₁ and λ ₂ are about 1.55 μm, ΔL is 0.8 mm to 82 mm from the equation ( ₁ ). Since ΔL has a relatively large value in this way, the change in the refractive index of the quartz glass forming the arm due to the temperature (dN / dT is about 1 × 1)
0 ^-6 : N is the refractive index, T is the temperature, and the linear expansion coefficient (about 0.
35 × 10 ⁻⁶ / ° C.) and the wavelengths λ ₁ and λ _{2 of the} demultiplexed light change depending on the temperature.

As an example, FIG. 7 shows changes in wavelength loss characteristics with respect to temperature of a Mach-Zehnder optical circuit. In the figure, the horizontal axis represents wavelength and the vertical axis represents loss. The demultiplexing interval Δλ is 0.1 nm. For example, the ambient temperature is 10 ℃
As it rises, the wavelength loss characteristic becomes 0.02 nm on the long wavelength side.
shift.

Therefore, in the conventional Mach-Zehnder optical circuit as shown in FIG. 5, a current is caused to flow through a heater having a metal film made of Cr or the like formed on one arm, and the heat generated at this time causes the multiplexing / demultiplexing. It was necessary to control the wavelength.

[0012]

However, the above-described conventional Mach-Zehnder optical circuit is not economical because it requires power supply to heat the heater to control the wavelength of the multiplexed / demultiplexed light. In particular, when a large number of Mach-Zehnder optical circuits are integrated, the power consumption becomes tens of watts, which makes it difficult to use in a practical system.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and to provide a Mach-Zehnder optical circuit which can stably perform wavelength multiplexing / demultiplexing with respect to temperature and which does not require heater heating.

[0014]

In order to achieve the above object, the present invention has a structure in which a core having a rectangular cross section and a clad having a refractive index smaller than that of the core are embedded around the core formed on a substrate. Between the directional couplers, a directional coupler composed of two parallel channel waveguides and a phase shift part consisting of two waveguides of different lengths is formed between the directional couplers. In the connected waveguide type Mach-Zehnder optical circuit, a negative refractive index material made of a material having a negative temperature coefficient of refractive index is provided at least on the waveguide of the phase shift section.

In addition to the above structure, the present invention is such that the thickness of the clad between the core and the negative refractive index material is between 0 μm and 20 μm.

In addition to the above structure, in the present invention, an oxide such as TiO ₂ may be used as the negative refractive index material.

In addition to the above structure, the present invention may use a photopolymer as the negative refractive index material.

In the present invention, in addition to the above constitution, multi-component glass may be used as the negative refractive index material.

[0019]

According to the above structure, since the temperature coefficient of the refractive index of the waveguide of the phase shift portion is positive and the temperature coefficient of the negative refractive index material provided on the waveguide is negative, the temperature rises. Even if the temperature drops, the temperature coefficients cancel each other and the temperature coefficient becomes very small as a whole, and the operation of the optical circuit becomes stable against temperature changes. Therefore, it is not necessary to heat the waveguide with the heater.

[0020]

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a plan view of an embodiment of a Mach-Zehnder optical circuit of the present invention, and FIG. 2 is a sectional view taken along line BB thereof. The same reference numerals are used for the same members as in the conventional example.

As shown in FIGS. 1 and 2, a buffer layer 22 is formed on a substrate 21 made of quartz (or Si), and two buffer layers 22 are made up of cores 23a and 23b and a clad 24. Input waveguides 11a, 11b of
Are formed. Both the input waveguides 11a and 11b are respectively connected to the input ends of the input-side directional coupler 13a, and the output ends of the input-side directional coupler 13a have two arms 15 whose lengths differ by ΔL, 16 are connected.

A negative refractive index material 25 made of photopolymer is provided on one arm 15. The negative refractive index material 25 is inserted into a groove obtained by etching a part of the clad 24. Reactive etching was used as the etching method. The phase shifter 14b is formed by the arms 15 and 16 and the negative refractive index material 25.

The temperature coefficient of the refractive index of the negative refractive index material 25 (dN
/ DT: N is the refractive index, T is the temperature) is -1 × 10 ^-4, and the temperature coefficient of the waveguide 23a (11a) made of quartz glass (dN /
The value is two orders of magnitude larger than dT) 1 × 10 ⁻⁶ .

Both arms 15 and 16 are connected to the input ends of the output side directional coupler 13b, and are connected to the output side directional coupler 13b.
The output ends of are connected to the two output waveguides 12a and 12b, respectively.

Next, the operation of the embodiment will be described.

The change of the wavelength loss characteristic with respect to the temperature of the Mach-Zehnder optical circuit is caused by the change of the refractive index of the silica glass forming the arms 15 and 16 (dN / dT is about 1 × 10 5).
^-6 : N is refractive index, T is temperature) and linear expansion coefficient (about 0.3)
5 × 10 ⁻⁶ / ° C.), the phase difference Φ between the two arms 15 and 16 in the phase shift unit 14b changes. This change is given by the equation (2).

[0028]

[Equation 2] Here, n _eff : Equivalent refractive index of the arm ΔL: Waveguide length difference between both arms λ: Wavelength T: Temperature The first term in the parentheses of the equation 2 is the coefficient of linear expansion of silica glass (about 0.35 × 10 ^-6 / ° C.), and the second term represents the change in the refractive index (thermo-optical effect) of quartz glass with temperature. In the conventional example, the sign of the first term and the sign of the second term are the same sign, and therefore change with temperature. When the shift amount of the wavelength loss characteristic in the conventional case is obtained by using Formula 2, the demultiplexing interval Δλ is 0.1 nm (ΔL is about 8300 μm).
m), when the temperature changes by 10 ° C., it becomes 0.02 nm, which is in agreement with the experimental result shown in FIG. 7.

In the case of this embodiment, the temperature coefficient of the refractive index (d
By providing the negative refractive index material 25 made of a material having a negative N / dT) on the arm 15, the first term and the second
Since the terms and can be canceled, stable operation with respect to temperature becomes possible.

That is, since the temperature coefficient of the refractive index of the waveguide of the phase shift section 14b is positive and the temperature coefficient of the negative refractive index material 25 provided on the arm 15 is negative, even if the temperature rises. Even if the temperature drops, the temperature coefficients cancel each other and the temperature coefficient becomes very small as a whole, and the operation of the Mach-Zehnder optical circuit becomes stable with respect to the temperature change. Therefore, there is no need to control the refractive index by heating the arm 15 with the heater 17 as in the conventional case.

Here, as an example, FIG. 3 shows wavelength loss characteristics with respect to temperature when a negative index material made of photopolymer is provided on the arm 15. In the figure, the horizontal axis represents wavelength and the vertical axis represents loss. In this case, the core 23a
The clad thickness t from the upper end to the lower end of the negative refractive index material 25 was 10 μm. Unlike the conventional one, the wavelength loss characteristic was hardly shifted even when the temperature was changed by 10 ° C., and stable operation could be confirmed.

Next, the optimum conditions will be described.

FIG. 4 shows the relationship between the cladding thickness and the wavelength loss shift amount when the temperature changes by 10 ° C. In the figure, the horizontal axis represents the cladding thickness, and the vertical axis represents the shift amount.

From the figure, it can be seen that when the cladding thickness is 20 μm or more, there is almost no effect even if the length L of the negative refractive index material 25 is increased. Further, it was confirmed that the amount of shift with respect to temperature can be made substantially "0" by optimizing the length of the portion where the negative refractive index material 25 is provided and the thickness of the cladding 24.

In this embodiment, a photopolymer is used as the negative refractive index material 25, but the material is not limited to this, and other materials such as oxides of TiO ₂ and multi-component glass having a negative temperature coefficient can be used. You may use the material of. Further, instead of providing the negative refractive index material on the waveguide, the substrate 21 itself forming the waveguide may be made of the negative refractive index material.

[0036]

In summary, according to the present invention, the following excellent effects are exhibited.

Since a negative refractive index material made of a material having a negative temperature coefficient of refractive index is provided at least on the waveguide of the phase shift portion, wavelength multiplexing / demultiplexing can be stably performed with respect to temperature, and heating by a heater is unnecessary. A Mach-Zehnder optical circuit can be realized.

[Brief description of drawings]

FIG. 1 is a plan view of an embodiment of a Mach-Zehnder optical circuit of the present invention.

FIG. 2 is a sectional view taken along line BB of FIG.

FIG. 3 is a diagram showing wavelength loss characteristics with respect to temperature when a negative refractive index material made of photopolymer is provided on an arm.

FIG. 4 is a diagram showing the relationship between the cladding thickness and the wavelength loss shift amount when the temperature changes by 10 ° C.

FIG. 5 is a diagram showing a plan view of a conventional Mach-Zehnder optical circuit.

FIG. 6 is a sectional view taken along line AA of FIG. 5;

FIG. 7 is a diagram showing changes in wavelength loss characteristics with respect to temperature of a Mach-Zehnder optical circuit.

[Explanation of symbols]

 21 substrate 13a directional coupler (input side directional coupler) 13b directional coupler (output side directional coupler) 14b phase shift section 15, 16 waveguide (arm) 23a, 23b core (waveguide) 24 clad 25 Negative refractive index material

Claims (6)

[Claims] 1. A single mode channel waveguide having a structure in which a core having a rectangular cross section and a periphery of the core are buried on a substrate with a clad having a refractive index smaller than that of the core. In a waveguide type Mach-Zehnder optical circuit in which two directional couplers made of channel waveguides and a phase shift part made of two waveguides having different lengths are connected between the directional couplers, at least the phase A Mach-Zehnder optical circuit, wherein a negative refractive index material made of a material having a negative temperature coefficient of refractive index is provided on the waveguide of the shift portion. 2. The Mach-Zehnder optical circuit according to claim 1, wherein the thickness of the clad between the core and the negative refractive index material is between 0 μm and 20 μm. 3. The Mach-Zehnder optical circuit according to claim 1, wherein a material having a refractive index smaller than that of the core of the waveguide is used as the negative refractive index material. 4. The Mach-Zehnder optical circuit according to claim 1, wherein an oxide such as TiO _{2 is} used as the negative refractive index material. 5. The Mach-Zehnder optical circuit according to claim 1, wherein a photopolymer is used as the negative refractive index material. 6. The Mach-Zehnder optical circuit according to claim 1, wherein a multi-component glass is used as the negative refractive index material.