JP2784496B2 - Waveguide type optical isolator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a waveguide type optical isolator, and more particularly, to an optical isolator for preventing unnecessary reflected light from returning to a laser or the like in the field of optical communication. It is about.

(Prior Art and Problems) Conventionally, as shown in FIG. 4, this type of apparatus has been put to practical use only in what is called a bulk type in which individual parts are combined.

Such a bulk-type optical isolator is composed of two polarizers 1 and 1 'whose axes are inclined at 45 degrees to each other and a non-reciprocal polarizer sandwiched therebetween.
It is constituted by a magneto-optical crystal 2 to which a magnetic field H functioning as a 45-degree polarization plane rotating element is applied. Magneto-optical crystal 2
4 transmits light only in the direction of the light beam 3 and the component shown in FIG. 4 functions as an optical isolator.

However, this type of optical isolator is expensive because the magneto-optical crystal 2 and the polarizers 1 and 1 'are expensive, and the overall dimensions are large because the optical isolator is connected to a fiber or the like using a lens. For example, there are drawbacks such as a large size of about 20 mm × 9 mm × 9 mm and difficulty in alignment when combined with a lens and a fiber.

It has been proposed to make the magneto-optical crystal 2 into a film having a thickness of about 200 μm in order to partially improve these disadvantages, and it has been confirmed that the overall dimensions can be reduced to about 6 mm × 5 mm × 5 mm. However, even if such an improvement is made, it is still essentially a bulk type, and the drawbacks such as difficulty in alignment have not been solved.

In recent years, research and development of optical transmission systems has been remarkable, and the conversion of optical components into waveguides has been studied.
There is a strong demand for a waveguide-type isolator that can be directly coupled to a waveguide-type optical component such as a fiber.
By the way, in the waveguide, unlike the bulk crystal, except for the two types of waveguide modes perpendicular to each other, TE and TM, it is not allowed as an eigenmode. Therefore, when configuring a waveguide type isolator, a polarizer as shown in FIG. Cannot be 45 degree arrangement. Therefore, as a component of the waveguide type isolator, in addition to a pair of polarizers in the same axial direction, a non-reciprocal TE-TM mode converter corresponding to a bulk type non-reciprocal 45 degree rotating element, and a reciprocal 45 degree rotating element A functioning reciprocal TE-TM mode converter is required.

As an attempt to construct such a waveguide type isolator, as shown in FIG. 5, a substrate 4, a magneto-optical crystal thin film non-reciprocal mode converter (magnetization M is parallel to the traveling direction of light) 5, a magneto-optical device A crystal thin film reciprocal mode converter (magnetization M is perpendicular to the traveling direction of light and forms an angle of Θm with a normal 7 of the film surface) 6; a metal thin film 8 as a mode filter;
8 'has been proposed (Japanese Patent Laid-Open No. 58-1983).
-221810). This configuration is characterized in that the reciprocal mode converter 5 uses magnetic birefringence. However, since it is difficult to form the non-reciprocal mode converter 5 and the reciprocal mode converter 6 having different directions of the magnetization M on the same magneto-optical thin film, the light having the configuration shown in FIG. Isolation ratio of isolator is 13dB
Is the best (K. Ando et al. Applied Physics Letter
Vol.53, No.1, p.4, 1988.). In addition, the waveguide type isolator having the configuration shown in FIG. 5 has a two-dimensional waveguide structure that does not confine guided light in an in-plane direction, and operates only in the zero-order mode among several modes that can propagate through the waveguide. For this reason, light can be introduced only by a limited method such as the prism coupler method. In addition to the fact that the core diameter of the optical fiber and the thickness of the waveguide layer are too different, it cannot be practically used because it cannot be directly connected to an optical fiber or the like.

As an attempt to match the thickness of the waveguide layer with the core diameter of the fiber, it has been proposed to make the magneto-optical thin film a two-layer structure and provide a cladding layer having a slightly lower refractive index under the waveguide layer. It has been confirmed that the thickness of the layer can be made approximately the same as the core diameter of the fiber (Japanese Patent Publication No. 62-57014). However, there is no change in that it basically has a two-dimensional waveguide structure and cannot be connected to a fiber or the like.

On the other hand, recently, it has been proposed to form a waveguide type optical phase plate by providing a stress release groove in a part of a cladding layer near a core portion of a buried waveguide to which a stress has been applied (Japanese Patent Application Laid-Open No. 63-1988). No. 147114). It is possible to construct a reciprocal mode converter by using this waveguide type optical phase plate, but there is no example applied to a waveguide type optical isolator so far.

The present invention has been made in view of the above-described problems, and has as its object to provide a small-sized, low-cost, practical three-dimensional waveguide optical isolator in which all components are formed of a single-mode waveguide. I do.

(Means for Solving the Problems) In order to achieve the above object, a waveguide-type optical isolator according to the present invention has a quartz glass core portion embedded in a quartz glass clad layer formed on a substrate. Reciprocal mode conversion in which a stress release groove having a predetermined length is formed by removing a part of the cladding layer in the vicinity of the core portion along the core portion of the three-dimensional embedded quartz glass single mode waveguide. Part, a non-reciprocal mode converter comprising a buried single-mode waveguide of magnetic garnet, and loading a metal thin film on the cladding layer at the light input end and light output end of the quartz-based single mode waveguide. A non-reciprocal mode converter is formed between the reciprocal mode converter and one of the mode selectors in a fitting groove formed on the substrate simultaneously with the stress release groove. It is characterized in that it is mounted with the embedding layer downward and fixed after positioning.

 Hereinafter, the present invention will be described in detail with reference to the drawings.

(Structure and Operation of the Invention) FIGS. 1 (a), 1 (b), and 1 (c) show a basic structure of an example of the structure of the present invention. FIG. 1 (a) is a plan view of the structure example. FIG. 1B is a lateral side view, and FIG. 1C is a vertical side view. In the figure, reference numerals 9 and 9 'denote mode selectors, 10 denotes a non-reciprocal mode converter, and 11 denotes a reciprocal mode converter.

The mode selectors 9 and 9 'are formed on the cladding layer 12 of the three-dimensional buried type single mode waveguide by the metal thin film 8 represented by Al.
And operates to transmit the TE mode and absorb the TM mode.

The non-reciprocal mode converter 10 includes a magnetic garnet embedded waveguide 15 to which a magnetic field 17 in a direction parallel to the light beam 3 is applied by a coil or a magnet. When the Faraday rotation coefficient of the magnetic garnet thin film constituting the magnetic garnet embedded waveguide 15 is _{Ｆ F} (deg / cm), the waveguide length 19 satisfies the relationship of | Θ _F | · l = 45 °. It is designed to operate with TE-TM non-reciprocal 50% mode conversion. Also, a magnetic garnet embedded waveguide
Reference numeral 15 denotes a quasi-TM mode in which the main component of the electric field of the guided light is parallel to the quasi-TE mode perpendicular to the normal 7 of the film surface and is set to propagate in a single mode, and to maintain good linear polarization. The propagation constants are set so as to match. If the propagation constants of the two do not match, even if the optical power is converted to 50% mode, the phases of the two are shifted, which causes a deterioration in element characteristics. The magnetic garnet embedded waveguide 15 is fitted between the mode selector 9 and the reciprocal mode converter 11 using the guide block 16.

The reciprocal mode converter 11 includes a cladding layer 12 near the core 13.
The stress is released from the core portion 13 by the stress release groove 14 partially opened in the direction of the principal axis 18 of the birefringence perpendicular to the light propagation direction and from the normal direction 7 of the film surface as shown in FIG. 22.5 on groove 14 side
A waveguide-type half-wave plate is formed by setting the length of the stress release groove 14 to a predetermined length, and the TE-TM
It is set to function as a reciprocal 50% mode converter.

The operating principle of the waveguide type optical isolator of the present invention will be described with reference to FIGS. First, when the sign of the theta _F of the magnetic garnet will be described a positive (theta _F code direction is the same direction parallel to the traveling direction and the external magnetic field H of the light constituting the magnetic garnet embedded waveguide 15, The direction in which the electric field vector of light rotates clockwise as viewed from the light source side is defined as positive).

In FIG. 1, the guided light incident on the core from A is absorbed by the metal thin film 8 loaded on the waveguide by the mode selector 9, and only the quasi-TE mode is incident on the non-reciprocal mode converter 10.

In the non-reciprocal mode converter 10, the guided light is subjected to 50% mode conversion, converted into quasi-TE mode 50% and quasi-TM mode 50% guided light, and propagated to the reciprocal mode converter 11.

The guided light is converted to the quasi-TE mode 100% by the reciprocal mode converter 11, and is emitted to B through the mode selector 9 '.

Conversely, the guided light incident on the core 13 from B is absorbed by the mode selector 9 'in the quasi-TM mode, and only the quasi-TE mode is incident on the reciprocal mode converter 11.

In the reciprocal mode converter 11, the guided light is mode-converted by 50% and is converted into guided light in the quasi-TE mode 50% and the quasi-TM mode 50%.

Next, the guided light is converted into the quasi-TM mode 10 by the nonreciprocal mode converter 10.
Since it is converted to 0%, it cannot pass through the mode selecting section 9 and the guided light emitted to A becomes 0, and the optical circuit of FIG. 1 functions as an optical isolator.

(Embodiment) In this embodiment, an Al thin film was used as the metal thin films 8 and 8 'of the mode selectors 9 and 9' in FIG.

FIG. 2 shows a reciprocal mode converter 11 used in the embodiment of the present invention.
FIG. In this embodiment, a silicon substrate having a thickness of 0.7 mm is used for the substrate 4, and quartz glass is used for the core 13 and the cladding layer 12. The thickness of the clad layer 12 was 50 μm, and the center of the core 13 was 25 μm from the substrate surface. When connecting the magnetic garnet-embedded waveguide 15, the core was formed in the same square shape as the core of the magnetic garnet-embedded waveguide so as to reduce the connection loss. Further, for single mode propagation, the relative refractive index difference between the core 13 and the cladding layer 12 was set to 0.25%. The distance S from the center of the core 13 to the stress relief groove 14 was set to 35 μm in order to set the angle at which the principal axis angle of the birefringence of the core was inclined from the normal to the substrate surface to 22.5 degrees. In addition, in order to provide a function as a 50% mode converter at a used wavelength of 1.3 μm, the length L of the stress release groove 14 is set to 2.6 mm.
And The width T of the stress release groove 14 was 200 μm.

The structure of such an optical waveguide and a stress relief groove is SiCl ₄ , T
It can be formed by a well-known processing method by a combination of a glass film deposition technique by a fire hydrolysis reaction of a glass forming raw material gas such as iCl ₄ and a dry etching process represented by RIE.

FIG. 3 is a cross-sectional view of the magnetic garnet embedded waveguide 15 used in the embodiment of the present invention, in which 20 indicates a buried layer, and 21 indicates a lower cladding layer.

In the present embodiment, in order to make the propagation constants of the quasi-TE mode and the quasi-TM mode equal, the shape of the core 13 is rectangular, and the buried layer 20
And the lower cladding layer 21 have the same refractive index. The width W of the core 13 was 8 μm.

In order to fabricate the magnetic garnet waveguide, for example, a two-layer garnet thin film is formed on the substrate 4 by an LPE method, a sputtering method or the like, and the core is patterned by an etching method such as ion milling and reactive ion etching (RIE). Finally, it can be manufactured by depositing a buried layer by an LPE method, a sputtering method or the like.

As a specific configuration of the magnetic garnet embedded waveguide 15, a substrate 4 is made of a GGG (gadolinium gallium garnet) substrate having a thickness of 0.5 mm. Yttrium iron garnet) was used. Since the composition of the substitutional YIG is desirably a composition that provides in-plane magnetization in order to reduce the external magnetic field 17 necessary for the device to operate, the (Sc, Ga) YIG
And (La, Ga) YIG.

The refractive indices of the core portion 13, the buried layer 20, and the lower cladding layer 21 are changed by changing the Ga substitution amount so that the core portion 13 propagates in a single mode and the width W is 8 μm.
Set to 2.199. Magnetic garnet embedded waveguide 15
Theta _F is used wavelength 1.3μ's element length l of these substituted YIG
Since it is about 150 deg / cm in m, it was set to about 3.0 mm so as to operate as a 50% mode conversion unit.

The magnetic garnet embedded waveguide 15 thus obtained
Is embedded in the single mode optical waveguide having the reciprocal mode converter 11 and the mode selectors 9 and 9 'shown in FIG.
The waveguide-type optical isolator shown in FIG. 1 was constructed by mounting along the guide block 16 with the 20 facing down, and positioning and fixing with an adhesive. The external magnetic field 17 is the same as the displacement YIG used in the magnetic garnet embedded waveguide 15.
Is applied from A to B direction for the sign of the theta _F is positive.

The overall size of the waveguide type optical isolator of this embodiment is about 15.6 mm in element length (mode selector section 9, 9 'about 50 mm ×
2 + magnetic garnet embedded waveguide 15 about 3.0 mm + reciprocal mode converter 11 2.6 mm) × width 0.5 mm × thickness 1.25 mm, which is smaller than the bulk type.

The isolation ratio of the waveguide type isolator of this example was actually measured by connecting to an optical fiber. The isolation ratio of the waveguide type isolator of this embodiment is saturated at the number of magnetic fields + Oe and shows a saturation value of about 20 dB. The saturation value of the isolation ratio of 20 dB is larger than the conventional maximum isolation ratio of 13 dB of the conventional optical isolator shown in FIG. Also, the magnetic field required to saturate the isolation ratio is two orders of magnitude smaller than the magnetic field number kOe required to saturate the bulk optical isolator.

Incidentally, the material of the buried layer 20 of the magnetic garnet buried waveguide 15 is the lower clad layer in the above embodiment.
Although the same substitution type YIG as 21 was used, similar effects were obtained by using an amorphous material such as Ta ₂ O ₅ or Nb ₂ O ₅ whose refractive index was made equal to that of the lower cladding layer 21.

(Effects of the Invention) As described above, the waveguide type isolator of the present invention uses a waveguide type half-wave plate using a stress release groove which can be easily formed in a reciprocal mode converter by a known processing method. The fabrication method is easier than the conventional waveguide type isolator using magnetic birefringence for the reciprocal mode converter that requires complicated magnetic anisotropy control, and the amount of reciprocal mode conversion is easily adjusted. Therefore, there is an advantage that a high isolation ratio can be obtained. In addition, since it has a three-dimensional waveguide structure and operates in a single mode, there is an advantage that it can be directly connected to another optical circuit element for use. In addition, the waveguide type isolator of the present invention operates sufficiently with a magnetic field having a magnitude several orders of magnitude lower than that of a normal bulk type isolator, and thus has the advantage of reducing the size and cost of the device.

[Brief description of the drawings]

FIG. 1 is a three-view drawing showing the basic structure of the present invention, FIG. 2 is an enlarged view of a reciprocal mode converter used in the embodiment of the present invention, and FIG. 3 is a magnetic garnet embedded in the embodiment of the present invention. FIG. 4 is a perspective view showing a configuration of a conventional bulk optical isolator, and FIG. 5 is a castera.
FIG. 1 is a perspective view showing a configuration of a waveguide type optical isolator proposed by the present inventors. 1, 1 '... polarizer, 2 ... magneto-optical crystal, 3 ... light beam, 4 ... substrate, 5 ... magneto-optical crystal thin film non-reciprocal mode converter, 6 ... magneto-optical crystal thin film reciprocal mode converter , 7
... normal to film surface, 8, 8 '... metal thin film, 9, 9' ...
Mode selector section, 10 ... Non-reciprocal mode conversion section, 11 ...
... Reciprocal mode converter, 12... Clad layer, 13... Core, 14... Stress release groove, 15... Magnetic garnet embedded waveguide, 16. Guide block, 17 ... a magnetic field parallel to the light beam 3 applied by a coil or magnet, 18
... the birefringent principal axis direction of the core part in the reciprocal mode converter, 19 ... the element length l of the magnetic garnet embedded waveguide 15, 20 ... the buried layer, 21 ... the lower clad layer.

Continuation of the front page (56) References JP-A-58-221810 (JP, A) JP-A-63-147114 (JP, A) JP-A-64-19309 (JP, A) (58) Fields investigated (Int) .Cl. ⁶ , DB name) G02B 6/12

Claims (1)

(57) [Claims] 1. A three-dimensional buried quartz glass single mode waveguide in which a quartz glass core is buried in a quartz glass clad layer formed on a substrate. A reciprocal mode converter in which a stress release groove of a predetermined length is formed by removing a part of the clad layer in the vicinity, a non-reciprocal mode converter including a buried single mode waveguide of magnetic garnet, A mode selector formed by loading a metal thin film on the cladding layer at the light incident end and light output end of the quartz-based single mode waveguide, wherein the non-reciprocal mode converter is the reciprocal mode converter. A waveguide type, which is mounted on a fitting groove formed on the substrate at the same time as the stress release groove between one of the mode selection portions and with a buried layer facing down and fixed after alignment. Light Isolator.