JP3327937B2 - Waveguide type polarization control element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization control element for converting and controlling the polarization state of incident light by changing the phase difference between two orthogonal polarization components using an optical member having birefringence. .

[0002]

2. Description of the Related Art In recent years, attention has been paid to systems using light, and research and development of optical systems have been actively conducted in various fields. In developing an optical system, it is important to make good use of the characteristics of the light used.
One useful property of light is polarized light. Polarized light refers to light whose electric field vector of a light wave vibrates or rotates with a certain regularity, and includes linearly polarized light, circularly polarized light, and elliptically polarized light. By successfully controlling and utilizing these polarization states, various useful optical systems can be constructed. A magneto-optical disk system or the like that records and reproduces information by utilizing polarization rotation by the optical Kerr effect can be said to be a typical example.

A polarization control element represented by a retardation plate is indispensable in configuring an optical system utilizing the above-described polarization characteristics of light. A number of reference books have been published on polarization control elements that have been generally used in the past. For example, the book is described in “How to Use Optical Components and Points to Consider” (Tetsuo Sueda, Optronics). As an example, a phase difference plate or a Babinet corrector as shown in FIG. 9 can be given as an example. In both cases, a birefringent material having a different refractive index depending on the polarization direction of propagating light is used to give a phase difference between two orthogonal linearly polarized light components to control the polarization state.

In the quarter-wave retarder shown in FIG. 1A, a phase difference between a light wave oscillating in the z-direction and a light wave oscillating in the x-direction due to the propagation of the birefringent material is π / 2. A birefringent material cut out to such a thickness has a function of changing incident linearly polarized light inclined at 45 ° from the z direction into circularly polarized light as shown in the figure.

On the other hand, a phase difference of π can be given by making the thickness to be cut out twice the thickness of the quarter-wave retardation plate. This has a function of rotating the polarization direction of the incident linearly polarized light by 90 °, and is called a 波長 wavelength retardation plate.

The phase difference plate as described above has a fixed phase difference. The Babinet compensator shown in FIG. 1B can adjust the phase difference. In this method, two birefringent crystals are polished in a wedge shape perpendicular to the optical axis and arranged as shown in the figure. The two wedges are not fixed, but are configured to be able to move with an apparent thickness change, such as a micrometer head.
Therefore, the phase difference can be changed by micro screw feed, and as shown in the figure, linearly polarized light can be changed to circular or elliptically polarized light,
It has a general-purpose polarization control function such as rotating the polarization direction by 90 °.

[0007]

All of the above-mentioned phase control elements are bulk-type optical elements, which are a major obstacle in reducing the size of an optical system. In particular, when used in an integrated ultra-small optical system using an optical waveguide or a network using an optical fiber, it may be a hindrance to integration of the entire system or a cause of a decrease in overall flexibility.

Further, since the birefringence of the material is used,
Since it is necessary to cut out and polish a birefringent crystal and to control its thickness with the accuracy of the order of the wavelength of light, there is a problem that it is costly to obtain a high-quality crystal.

In a variable phase difference element such as a Babinet corrector, the method is mechanical, and there is a problem that the method is not suitable for applications such as electrically controlling the phase difference.

The waveguide type polarization control element of the present invention has been made in view of such a problem, and an object thereof is to provide an optical waveguide having a multilayer thin film structure in which the sign of the temperature coefficient of the refractive index differs. It is an object of the present invention to provide a waveguide type polarization control element which is configured by laminating optical media and can freely control a phase difference between two orthogonal electromagnetic wave components of propagating light.

[0011]

According to a first aspect of the present invention, an optical member having birefringence is used to change the phase difference between polarized light components in two orthogonal directions. Using a polarization control element for converting and controlling the polarization state of light, the optical member has an optical waveguide structure, and the optical waveguide structure includes a plurality of optical media having a relatively high refractive index and an optical medium having a low refractive index. In an optical waveguide type polarization control element having a multilayer thin film structure provided with structural birefringence by playing, the optical waveguide having the multilayer thin film structure is formed by laminating optical media having different signs of the temperature coefficient of the refractive index. Have been. According to a second aspect, in the polarization control element according to the first aspect, the polarization control element further includes a heater for causing a temperature change in an optical material forming the optical waveguide having the multilayer thin film structure.

[0012]

According to the first aspect of the present invention, the optical member having birefringence has an optical waveguide structure, and the optical waveguide structure is an optical waveguide having a multilayer thin film structure. The layers are laminated from optical media having different signs of the rate temperature coefficient. Further, the second invention has a heater for causing a temperature change of the optical medium according to the first invention.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an optical waveguide having a multilayer thin film structure, which is the most important component of the present invention, and its thermo-optical characteristics will be briefly described.

FIG. 1A is a schematic view of a multilayer thin film optical waveguide. A large number of media a2 and media b3 having different refractive indices are alternately laminated on a substrate 1 made of a transparent optical medium, and the upper surface is covered with an upper cladding layer 4. When the volume average refractive index of the entire laminated portion is larger than that of the substrate 1 or the upper cladding layer 4 and the thickness of each layer constituting the laminated portion is sufficiently shorter than the wavelength of light, the laminated portion has a structural property. , That is, the multilayer thin-film optical waveguide layer 5 in the figure. FIG. 1B shows the refractive index distribution at this time.

An optical waveguide having the above-described structural birefringence has mode propagation characteristics different from those of an optical waveguide having a single-layer waveguide layer. FIG. 2 schematically shows the mode propagation characteristics of the two. (A) shows the mode propagation characteristics of the single-layer optical waveguide, and (b) shows the mode propagation characteristics of the multilayer thin-film optical waveguide. In the case of an optical waveguide composed of a single waveguide layer, as shown in FIG. 7A, there is no large difference in the effective refractive index between the TE mode and the TM mode. Therefore, even if both TE and TM modes are simultaneously introduced and propagated in such an optical waveguide, there is almost no phase difference between them.

On the other hand, in the multilayer thin film optical waveguide, (b)
As shown in (2), due to the structural birefringence of the waveguide layer, the effective refractive indices for two orthogonal electromagnetic waves, TE mode and TM mode, are significantly different. Therefore, the phase difference generated between the two modes due to the propagation in the waveguide is much larger than that in the case (a) at the same propagation distance in the waveguide.

What determines the magnitude of the phase difference between modes due to propagation is the effective refractive index difference between the two as described above. The effective refractive index difference between modes of the symmetric multilayer thin-film optical waveguide is approximately expressed by the following equation (1) when the thickness of each layer is sufficiently shorter than the wavelength of light and the number of stacked layers is large.

[0018] _{N TE -N TM = (f 1} f 2 (n 1 2 -n 2 2) 2) / (f 1 n 2 2 + f 2 n 1 2) ... (1) ( However, f ₁ = t ₁ / (T ₁ + t ₂ ), f ₂ = t ₂ /
(T ₁ + t ₂ ), t ₁ and t ₂ are medium a and medium b, respectively.
Of a single layer). From the equation (1), it can be seen that the greater the refractive index of each laminated medium is, the more structural birefringence can be obtained. Using the characteristics of such a multilayer thin film optical waveguide,
A waveguide-type device having a function equivalent to that of a conventional polarization control element using a birefringent crystal can be configured.

By the way, it is convenient if the magnitude of the structural birefringence as described above, that is, the effective refractive index difference between the TE mode and the TM mode is externally changed, and thereby the phase difference between the modes can be controlled. is there. For that, (1)
As is clear from the formula, it is sufficient to change the difference in the refractive index of each medium to be laminated. Therefore, the present invention is devised so that the above-mentioned mode effective refractive index difference can be effectively changed by utilizing the refractive index change due to the thermo-optic effect. Hereinafter, the principle will be described with reference to the drawings.

Generally, the refractive index of an optical medium has a temperature dependence, and depending on the material, there are two types of materials, one having a positive temperature coefficient of refractive index as shown in FIG. 3A and the other having a negative temperature coefficient as shown in FIG. is there. When the above-mentioned multilayer thin-film optical waveguide is formed as shown in FIG. 4A using materials having different refractive index-temperature characteristics, n ₁ , n ₂ shifts in the opposite directions to the increase and decrease, respectively. As a result, as shown in FIG. 4B, the difference between the effective refractive indices of the TE mode and the TM mode greatly changes.

Therefore, in such a multilayer thin film optical waveguide,
By changing the temperature, the effective refractive index difference between the TE mode and the TM mode can be effectively controlled. FIG. 5A shows a current supplied from the outside by combining a multilayer thin-film optical waveguide having a large temperature dependency of the TE / TM mode effective refractive index difference and a heater based on the principle of the present invention. This constitutes an element for controlling the polarization by the method. In the figure, on a substrate 1,
A multilayer thin-film optical waveguide layer 5 composed of materials having different refractive index temperature coefficient signs is provided, and an upper clad layer 4 covers the multilayer thin-film optical waveguide layer 5. Further, a thin-film heater 6 for heating the multilayer thin-film optical waveguide layer 5 via the upper clad layer 4 is provided, and a power source 7 and a switch 8 are connected to the thin-film heater 6 to supply or cut off current, or By properly adjusting the amount of current by a current adjusting circuit (not shown), the TE / TM mode effective refractive index difference of the multilayer thin-film optical waveguide layer 5 is greatly changed, and the phase difference between two orthogonal electromagnetic wave components of the propagating light can be freely adjusted. Can be controlled.

For example, when linearly polarized light of 45 ° ascending upward as shown in the lower left side of FIG. 5A is introduced, the thin film heater 6 is set so that the phase difference between modes accompanying propagation in the waveguide becomes π.
If the temperature is adjusted by the amount of current, the light is emitted as linearly polarized light of 45 ° rising to the left as shown on the upper right side. By appropriately setting the temperature of the thin film heater 6 and controlling the phase difference between the modes, the light can be converted into circularly polarized light as shown in FIG. 5B or elliptically polarized light as shown in FIG. Such behavior is
The function of the above-mentioned Babinet corrector is electrically realized. FIG. 6 shows a first embodiment of the present invention.

Since the basic configuration is the same as that of FIG. 5, the basic functions are omitted. This embodiment is characterized in that a condensing lens 9 for introducing light into the multilayer thin-film waveguide layer and a collimator lens 10 for converting emitted light into parallel light are provided as one unit surrounded by a dotted line. is there. By doing so, it becomes a small and light electric control type phase control unit, which is useful when used in a conventional bulk type optical system or the like.

Next, a second embodiment is shown in FIG. This is an example in which light is introduced into the multilayer thin-film optical waveguide layer 5 using direct coupling of the polarization-maintaining fiber 11. As in the first embodiment, the vicinity of the multilayer thin-film optical waveguide layer 5 is unitized. Since this unit can be extremely miniaturized, it greatly helps to improve the degree of freedom of the system configuration when applied to a system using an optical fiber.

FIG. 8 shows a third embodiment. This is because the semiconductor laser 12 is connected to the multilayer thin film optical waveguide as shown in FIG.
It was joined at 5 ° from the end face. By doing so, it is possible to configure a polarization variable LD light source that can freely change the polarization state of the light emitted from the opposite end face of the multilayer thin-film optical waveguide layer 5 by an external current. Such a light source will be very useful in various system configurations using polarized light in the future.

[0026]

As described in detail above, according to the present invention,
Since the optical waveguide having the multilayer thin film structure is formed by laminating optical media having different signs of the temperature coefficient of the refractive index, it is possible to freely control the phase difference between two orthogonal electromagnetic wave components of propagating light. A waveguide polarization control element can be provided.

[Brief description of the drawings]

FIG. 1A is a diagram schematically illustrating a multilayer thin-film optical waveguide, and FIG. 1B is a diagram illustrating a refractive index distribution thereof.

FIG. 2A is a diagram schematically showing a mode propagation characteristic of a single-layer optical waveguide, and FIG. 2B is a diagram showing a mode propagation characteristic of a multilayer thin-film optical waveguide. .

FIG. 3A is a diagram showing a characteristic of a positive refractive index temperature coefficient, and FIG. 3B is a diagram showing a characteristic of a negative refractive index temperature coefficient.

FIG. 4A is a view showing a state in which light rays are incident on a multilayer thin film optical waveguide made of materials having different refractive index temperature characteristics, and FIG. 4B is a view showing a TE mode and a TM mode. FIG. 9 is a diagram showing how the difference in the effective refractive index of the negative electrode changes.

5A is a diagram showing a configuration of an element for controlling polarization by a current supplied from the outside, and FIG.
Is a diagram showing circularly polarized light, and FIG. 5C is a diagram showing elliptically polarized light.

FIG. 6 is a diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 7 is a diagram showing a configuration of another embodiment of the present invention.

FIG. 8 is a diagram showing a configuration of another embodiment of the present invention.

FIGS. 9A and 9B are diagrams illustrating examples of a conventional polarization control element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Medium a, 3 ... Medium b, 4 ... Upper clad layer, 5 ... Multilayer thin film optical waveguide layer, 6 ... Thin film heater, 7 ... Power supply, 8 ... Switch.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/00-1/125 G02B 5/30

Claims (2)

(57) [Claims] 1. A polarization control element for converting and controlling the polarization state of incident light by changing the phase difference between polarization components in two orthogonal directions using an optical member having birefringence. having an optical waveguide structure, the multi-layered film structure granted the birefringence of structural by the optical waveguide structure is more product layer and the optical medium of the optical medium and a low refractive index of the relatively high refractive index The waveguide-type polarization control element, wherein the optical waveguide having the multilayer thin film structure is formed of a stack of optical media having different refractive index temperature coefficients. 2. The waveguide type polarization control element according to claim 1, further comprising a heater for causing a temperature change in an optical material constituting the optical waveguide having the multilayer thin film structure.