JPH03296004A - Waveguide type optical isolator - Google Patents

Abstract

PURPOSE:To obtain the small-sized wavelength multiplex isolator without polarization dependency by composing the multiplex optical isolator of wavelength multiplexing and demultiplexing parts, mode selection parts, a nonreciprocal mode conversion part, and a reciprocal mode converter. CONSTITUTION:This optical isolator is equipped with the wavelength multiplexing and demultiplexing parts 21 and 21' formed at the light incidence end and light projection end of quartz glass single-mode waveguides 19 and 19', two optical directional couplers 18 - 18''' formed in the quartz glass single- mode waveguides 19 and 19', and mode selection parts 7 and 7' composed of two single-mode waveguides. Further, the isolator is equipped with the nonreciprocal mode converter 8 composed of a single-mode waveguide of magnetic garnet sandwiched between the mode selection parts 7 and 7' and the reciprocal mode converter 9 which has a stress release groove 12 formed in part of a clad layer 10 to specific length along a core part 11. Consequently, the optical isolator which is small in size and inexpensive and operates with plural wavelengths at the same time and has no polarization dependency is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical isolator for preventing unnecessary reflected light from returning to lasers, optical amplifiers, etc. in the field of optical communications.

(Prior Art) Optical isolators are important elements used in the field of optical communications for purposes such as stabilizing laser light sources, suppressing line echoes, and preventing noise generation in optical amplifiers. Conventionally, this type of element has been put into practical use as a so-called bulk type element, which combines a magneto-optic crystal 2 and two polarizers 1 and 1', as shown in FIG. In particular, optical isolators that do not depend on the polarization direction of signal light used in coherent communications, etc., use polarization-independent optical isolators that use a birefringent crystal wedge-shaped prism as a 1°1' polarizer. .

However, in this type of optical isolator, the magneto-optic crystal 2 and polarizers 1 and 1' are expensive, so the optical isolator itself is also expensive, and since it is coupled to a fiber etc. using a lens, the overall size However, for example, in the case of a polarization-independent optical isolator, there are drawbacks such as a large size of about 70 m++ x 15 mm x 15 m, and difficulty in alignment when combining with lenses and fibers.

Recently, in the field of optical communications, wavelength division multiplexing communication, which multiplexes the wavelengths of signal light transmitted within an optical fiber, has been proposed for the purpose of increasing the amount of information transmitted through a single optical fiber. Furthermore, in order to increase the non-repeater transmission distance, a communication system using an Er-doped fiber amplifier has been proposed, which amplifies the intensity of the signal light by propagating the signal light together with pump light of different wavelengths in an Er-doped fiber. . These new communication systems require wavelength multiplexing optical isolators that operate in two or more wavelength ranges simultaneously. In particular, communication systems using Er-doped fiber amplifiers require polarization-independent and wavelength-multiplexing optical isolators. However, in the optical isolator shown in FIG. 10, the magneto-optic properties of the garnet crystal used in the magneto-optic crystal 2 have wavelength dependence, so the elements of the magneto-optic crystal 2 necessary for the device to operate at each wavelength are Long! ′ are different. Therefore, in one optical isolator, for example, signal wavelengths used in wavelength division multiplexing communication systems, such as the 0, 8μ wavelength band, 1.3μ fog band, and 1.5μ wavelength band, and Er doped fiber amplifiers can be used. Excitation light wavelength 0.98μ carrier band and signal light wavelength 1 used in the communication method used
．． It has a drawback that it cannot simultaneously exhibit sufficient operation in two or more wavelength bands of the 5 μm band.

In order to realize a wavelength multiplexed optical isolator, the 10th
A wavelength demultiplexing element is formed in front of the optical isolator shown in the figure, and the incident light is separated into wavelengths and input into a single wavelength optical isolator that is set to operate optimally for each wavelength. A configuration may be adopted in which multiplexing is performed using a wavelength multiplexing element formed at the output end of the single wavelength optical isolator. Such a wavelength multiplexing/demultiplexing element can be realized by combining an interference filter using a dielectric multilayer film or the like and a prism, and by using this, a wavelength multiplexing type optical isolator can be easily realized. However, such wavelength multiplexing optical isolators are bulk types that combine individual parts, so alignment when assembling individual parts is difficult, and since they are coupled to optical fibers using lenses, optical For example, when a bulk type optical isolator is used as a single wavelength type isolator, there is a problem that the overall size of the isolator is about 110 cm x 25 WIX x 15 cm. On the other hand, it has recently been proposed to fabricate a waveguide-type wavelength multiplexing/demultiplexing element by forming an optical directional coupler in a silica-based waveguide (N, Takat.

et al, Electronics Letter
s, vol, 22. No, 6+ p.

321). By using this waveguide-type wavelength multiplexing/demultiplexing element, it is possible to realize a waveguide-type wavelength multiplexing optical isolator, but there are no examples that have been applied to waveguide-type wavelength multiplexing optical isolators so far. There isn't.

(Problems to be Solved by the Invention) The present invention is a compact, low-cost, practically usable three-dimensional system in which all components are composed of single-mode waveguides, which operates simultaneously at multiple wavelengths, and has no polarization dependence. An object of the present invention is to provide a waveguide type optical isolator.

(Means for solving problems) FIG. 1 shows n signal light wavelengths λ1. λ2. .

..., shows the basic configuration of an n-wavelength multiplexing optical isolator that can support λ9, (A) is a plan view, (B) is a side view, (C
) is a view seen from the B side of Figure (A), and is similar to Figure 1 (A).
In the figure, 21 and 21' are wavelength multiplexing/demultiplexing sections, 7 and 7' are mode selection sections, 8 is a non-reciprocal mode converter, and 9 is a reciprocal mode converter.

The wavelength multiplexer/demultiplexer 21, 21' is constructed by connecting n-1 wavelength multiplexer/demultiplexer optical directional couplers 22 in series as shown in FIGS. 2(A) and 2(B). ing.

Optical directional coupler 22 for the n-th wavelength multiplexer/demultiplexer in FIG.
By adjusting the length of the coupling part and the distance between the two waveguides constituting the optical directional coupler, it functions as a 100% power shifter only for signal light of wavelength λゎ41,
It is set so that no power is transferred to signal lights of other wavelengths. As a result, the wavelength multiplexing/demultiplexing section 2L
21' is used as a wavelength demultiplexer, as shown in Fig. 2 (A
), the wavelength λ1 propagating through one waveguide
．． λg, +++.

n signal lights of λ7, λ, from the top of FIG. 2(A).

λ! , HHII, and λ indicate the function of demultiplexing into n waveguides in this order. Further, when used as a wavelength multiplexing section, as shown in FIG. 2(B), from the top of FIG. 2(B), λ1° λ2. . . . indicates a function of combining signal lights propagated through n waveguides in the order of λ7 into one waveguide.

The mode selection section 7.7', as shown in FIG. 1(A),
Optical directional couplers 18.1 for two mode selection units each
8', 18', 18'''' and a single mode waveguide section 19, 19'.
It is composed of n pieces arranged in parallel. These Matsuhatsu Ender interferometers have wavelengths λ in order from the top in FIG. 1(A).
3. λ2. ..., is set to function in λi. That is, in FIG. 1(A), the optical directional couplers 18, 18', 1 of the n-th Matsuhatsu Ender interferometer from the top
B", 18- is set to function as a 3 dB coupler by making the length of the coupling part half the coupling length at wavelength λ9. Also, the nth Matsuhatsu Ender interferometer from the top Single mode waveguide section 19.1
The two waveguides 9' are arranged so that the relative phase difference of the guided light propagating through each at wavelength λ7 is π for the quasi-TE mode in which the main component of the electric field is perpendicular to the normal 6 of the film surface. quasi-TM where the main component of the electric field is parallel to the normal 6 of the film surface.
The mode is set to 0. As a result, the n-th Matsuhatsu Ender interference needle from the top in FIG.
Works as shown in (), 100% for 7M mode
Functioning as a power shifter, the mode selector 7, 7' as a whole has wavelengths λhλ2 . . . . functions as a TE-TM momo demultiplexer and a TE-7M mode multiplexer for the λ7 signal light.

The non-reciprocal mode converter 8 converts the light beam 3 by a coil or magnet.
2D lines arranged in parallel to which a magnetic field 15 is applied in a direction parallel to
Magnetic garnet embedded waveguide 13 with book core
Consisted of. FIG. 4 is a diagram showing the structure of the magnetic garnet-embedded waveguide 13, and C and D in FIG. 4 correspond to C2D in FIG. 1 (^). 2D book core part 1
As shown in FIG. 4(A), 1 is divided into n groups of 2 to 0, each consisting of two adjacent wires. In FIG. 4, the n-th core group from the left has the Faraday rotation coefficient θF(λ, ) (deg/c11
), then the waveguide length l is 1θF(λ7)・f,
It is designed to satisfy the relationship of −45°, and the wavelength λ
7 to perform the TE-TM non-reciprocal 50% mode conversion operation. Therefore, the magnetic garnet embedded waveguide 13 as a whole has a wavelength of λ3. λ2. ...
, λi, operates as a TE-TM non-reciprocal 50% mode converter. In addition, the magnetic garnet embedded waveguide 13 is set so that each core portion propagates in a single mode at the corresponding wavelength λ, and in order to maintain good linear polarization, quasi-TE mode and quasi-TM mode are set. The mode propagation constants are set to match. If the propagation constants of the two do not match, even if the optical power is converted into a mode by 50%, the phases of the two will be shifted, resulting in deterioration of the device characteristics.

In addition, the magnetic garnet embedded waveguide 13 uses a guide block 14 to connect the mode selection unit 7 and the reciprocal mode change unit.
It is fitted between the replacement parts 9 with the board 4 facing upward.

5 shows the structure of the reciprocal mode converter 9, CA) is a plan view, (B) is a view seen from the B side of FIG.
C and D in Figure (A) correspond to C and D in Figure 1. The reciprocal mode converter 9 includes two parallel mode converters as shown in FIG.
Stress release grooves 12 are formed in a part of the cladding layer 10 near the D core parts 11, so that the direction of the principal birefringence axis 16 of the core part 11 is perpendicular to the light propagation direction and the film of the substrate. 22.5 on the stress release groove 12 side from the direction of the surface normal 6
By setting the stress release grooves 12 to predetermined lengths, the 2D waveguide type 2
A half-wave plate is formed, each with 50% TE-TM reciprocity.
It is configured to function as a mode converter. At this time, the 2D waveguide type half-wave plate is used as shown in Fig. 5 (A
) They are divided into n groups of two adjacent from the center left, from ■ to @, each with a wavelength of λ1. λz, HII,.

The length and width of the stress relief groove 12 and the distance between the core portion 11 and the stress relief groove 12 are set so that the stress relief groove 12 operates optimally at λ.

The operating principle of the waveguide type optical isolator of the present invention will be explained using FIG. For convenience of explanation, in FIG. 1(A), among the 2n waveguides arranged in parallel in the mode selection section 7, 7', the non-reciprocal mode conversion section 8, and the reciprocal mode conversion section 9, the terminal end of the waveguide is The one in which the waveform is coupled to the wavelength multiplexing/demultiplexing section 21.degree. 21' will be referred to as a waveguide a, and the one in which it is not coupled will be referred to as a waveguide. For convenience, we will explain the case where the sign of θ of the magnetic garnet constituting the magnetic garnet embedded waveguide 13 is positive (the sign of θ is when the traveling direction of the light and the direction of the external magnetic field 15 are parallel and in the same direction). , the direction in which the electric field vector of light rotates clockwise when viewed from the light source side is defined as positive). λ1. which is incident on the core from A in FIG. 1(A).
λ3. ..., guided light with n wavelengths of λa multiplexed is,
In FIG. 1(A), the wavelength multiplexing/demultiplexing unit 21 selects λ, λ2 .
. The guided light split into wavelengths is converted into quasi-T by the mode selection unit 7.
The E mode is separated into the waveguide a, and the quasi-TM mode is separated into the waveguide S, and the two enter the non-reciprocal mode converter 8. The guided light is mode-converted by 50% in each waveguide in the non-reciprocal mode converter 8, with 50% in quasi-TE mode and 50% in quasi-TM mode.
% of the guided light and propagated to the reciprocal mode converter 9. In the reciprocal mode converter 9, the guided light becomes 100% quasi-TE mode in waveguide a, and 100% quasi-TM mode in the waveguide.
% and enters the mode selection section 7'. In the mode selection section 7', 100% of the power of the quasi-TM mode in the waveguide A is transferred to the waveguide a, and the quasi-TE mode is output to the wavelength multiplexing/demultiplexing section 21'. Guided light beams of different wavelengths that are incident on the wavelength multiplexing/demultiplexing section 21' and proceeding through zero waveguides are multiplexed into one waveguide and output to the OB in FIG. 1(A).

Conversely, λ1. which is incident on the core part 11 from B. λ2.・、・
The guided light with n wavelengths multiplexed in wavelengths λ1 . λ2. ...,
The light is split into 0 waveguides for each wavelength in the order of λ7. The guided light separated by wavelength is separated into a quasi-TE mode in a waveguide a and a quasi-TM mode in a waveguide a in a mode selection section 7'.
The light enters the reciprocal mode converter 9. In the reciprocal mode converter 9, the guided light undergoes 50% mode conversion in each waveguide, and is converted into guided light with 50% quasi-TE mode and 50% quasi-TM mode.

Next, the guided light is transferred to the waveguide a in the non-reciprocal mode converter 8.
The waveguide is converted into 100% quasi-TM mode and 100% quasi-TE mode, respectively, and enters the mode selection unit 7. In the mode selection unit 7, the quasi-TM mode of the waveguide a transfers 100% power to the waveguide, so the quasi-TE mode and the quasi-TE mode that are originally guided through the waveguide are combined in the wavelength multiplexing/demultiplexing unit 2.
1, and the guided light emitted to A in FIG. 1(A) becomes O, and the optical circuit shown in FIG. 1(A) has a wavelength λ.

λ2. . . . functions as a polarization-independent optical isolator that operates at n types of wavelengths of λ7.

(Example) Hereinafter, as an example of the present invention, wavelengths of 1.3μ and 1.5μ are shown.
A waveguide type optical isolator that operates simultaneously for 5 μm of guided light will be described.

FIG. 6 (A-1), (B-1), (C-1) and (A-2), (B2), (C-2) are wavelength multiplexing/demultiplexing sections used in the embodiment of the present invention. 21. '21' is an enlarged view of the mode selection section 7.7', and FIGS. 7A and 7B are enlarged views of the reciprocal mode conversion section 9. In this example,
A silicon substrate with a thickness of 0.7 m was used for the substrate 4 in these parts, and quartz glass was used for the core part 11 and the cladding layer IO. The thickness of the cladding layer 10 was 50 μm, and the center of the core portion 11 was set at a height of 25 μm from the substrate surface. The core part 11 is a magnetic garnet embedded waveguide 1
In order to reduce the connection loss when connecting the waveguides 3 to 3, they were made into a rectangular shape of 8 μm square, which is the same as the core portion of the magnetic garnet embedded waveguide 13. Also, for single mode propagation, the core part 11
The relative refractive index difference between the cladding layer 10 and the cladding layer 10 was 0.25%.

In the wavelength multiplexing/demultiplexing section 21.21', the wavelengths are 1.3 μm and 1.3 μm.
By adjusting the length and distance of the coupling portion of the two waveguides of the optical directional coupler 22 so that it functions as a 55μm multiplexer/demultiplexer, the optical directional coupler 22 can function as a 1.3μm wavelength multiplexer/demultiplexer.
00% power shift and no power shift at all for guided light with a wavelength of 1.55 μm. In the mode selection section 7.7', the Matsuhatsu Ender interferometer composed of waveguides ■ and ■ in FIG. , the propagation constants of the quasi-TE mode and quasi-TM mode of waveguide a and waveguide S were set to be equal at a wavelength of 1.55 μ-. Similarly, in Fig. 6 (A-1), a Matsuhatsu Ender interferometer composed of waveguides ■ and ■ is quasi-TM at a wavelength of 1.3μ.
The waveguide O such that 100% power transfer of the mode occurs
The propagation constants of the quasi-TE mode and quasi-TM mode of the waveguide (1) and (2) were made equal at a wavelength of 1.3μ. In the reciprocal mode converter 9, in order to set the angle at which the main axis of birefringence of the core part 11 is inclined from the substrate surface normal 6 to 22.5 degrees, the angle shown in FIG.
The distance S from the center of the core portion 11 to the stress release groove 12 shown in B) was 35 μm. In addition, the width T of the stress release groove 15
is next to 200μ. In FIG. 7(A), the lengths Ll and L of the stress release groove 12 for the waveguides ■, ■ and O20 are 50% at the wavelength of 1.55 μm and the wavelength of 1.3 μm.
In order to function as a mode converter, 3.1
m and 2.6 cabinets. The structure of such an optical waveguide and stress release groove is 5iCj! ,.

It can be formed by a well-known processing method that combines a glass film deposition technique using a flame hydrolysis reaction of a glass-forming raw material gas such as T i Cl a and a dry etching process typified by reactive ion etching (RIE). can.

FIG. 8 is an enlarged view showing the structure of the magnetic garnet embedded waveguide 13 used in the embodiment of the present invention, where (A) is a plan view and (B) is a view from the B side of FIG. 1 (A). In the figure, 2
3 indicates a buried layer, and 24 indicates a lower cladding layer. In this embodiment, in order to equalize the propagation constants of the quasi-TE mode and the quasi-TM mode, the shape of the core portion 11 is made rectangular, and the refractive index of the buried layer 23 and the lower cladding layer 24 are made equal. Further, the width W of the core portion 11 was set to 8 μm. In order to produce this magnetic garnet waveguide, for example, a two-layer garnet thin film is formed on the substrate 4 by liquid phase epitaxy (LPE method), sputtering method, etc., and the core portion 11 is formed by ion milling.
It can be manufactured by patterning using an etching method such as reactive ion etching (RIE), and finally depositing the buried layer 23 using an LPE method, a sputtering method, or the like.

The specific configuration of the magnetic garnet embedded waveguide 13 includes a GGG (gadolium gallium garnet) substrate with a thickness of 0.5 mm as the substrate 4, a core part 11, an embedded layer 2
Substitution type YIG (yttrium iron garnet) was used for the 3 and lower cladding layers 24, respectively. The specific composition of substitution type YIG is as follows:
5, it is desirable to have a composition with in-plane magnetization, so (Sc.

Ga)YIG and (La,Ga)YIC; were used. The core portion 11 propagates in a single mode, and the width W is 8 μm.
The refractive indexes of the core portion 11, the buried layer 23, and the lower cladding layer 24 were set to 2.200, 2.199, and 2.199 by changing the amount of Ga substitution so that the following values were obtained. In FIG. 7(A), the waveguides ■, ■ of the magnetic garnet embedded waveguide 13 and the element lengths 11 and 12 of O20 are θ of these substitution type YIG.

160 at wavelength 1.55 p m and 1.3 pwr
deg/c+i and 206deg/cm, so
They were set to approximately 2.8 mm and 2.2 mm to operate as a 50% mode converter at each wavelength.

The magnetic garnet-embedded waveguide 13 thus obtained is transferred to the wavelength multiplexing/demultiplexing sections 21 and 21' and the mode selection section 7. 7′
And a buried layer 2 shown in FIG. 8 CB) is added to the single mode optical waveguide in which the reciprocal mode conversion section shown in FIG. 7 is formed.
3 facing down along the guide block 14 shown in FIG. 1(A), and after alignment, was fixed with adhesive to construct the waveguide type optical isolator shown in FIG. 9.

Note that the external magnetic field 15 is generated by the magnetic garnet embedded waveguide 1.
Since the sign of θ of substitution type YIG used in 3 is positive,
The voltage was applied in the direction from ■ to ■ shown in FIG.

The overall size of the waveguide type optical isolator in this example is:
Element length approximately 75.9 m (wavelength multiplexing/demultiplexing section 21.21' is 2
0m x 2, mode selection section 7.7' is approximately 15.0 x 2,
Non-reciprocal mode converter 8 is approximately 2.81, reciprocal mode converter 9 is 3.1 an) x width 5.0m + x thickness 1.25a
++, which is smaller than that of a bulk wavelength multiplexing optical isolator.

The isolation ratio of the waveguide type optical isolator of this example was actually measured by connecting it to an optical fiber. In the waveguide type optical isolator of this example, the isolation ratio is saturated at a magnetic field number of 100e, and its magnitude is 1.55 μm at a wavelength of 1.55 μm.
It has saturation values of 21 dB and 20 dB at haze and 1.3 μ-, showing a high isolation ratio at both wavelengths, and 2
Operates as a wavelength multiplexed optical isolator. Further, the magnetic field required to saturate the isolage ratio is two times smaller than the number of magnetic fields koe required to saturate the bulk optical isolator.

By the way, in the above embodiment, the same substitution type YIG as the lower cladding layer 24 was used as the material of the buried layer 23 of the magnetic garnet buried waveguide 13, but the refractive index was made equal to that of the lower cladding layer 24. Similar effects were obtained using amorphous materials such as Taxes and NbzOs. In addition, in the above embodiment, the guided light wavelength is 1.5
Although a 5μ ring and a 1.3μ ring were used, it has been confirmed that a wavelength multiplexing type optical isolator exhibiting similar performance can be realized by using the present invention for guided light of other wavelengths.

(Effects of the Invention) As explained above, the waveguide type optical isolator of the present invention, unlike the waveguide type optical isolators proposed so far, can operate simultaneously for guided light of multiple wavelengths. It has the advantage of being suitable for use in multiplex optical communications. In addition, since the operation does not depend on the polarization direction of the incident light, E
It has the advantage of being suitable for use in optical communication systems using r-doped fiber amplifiers. Furthermore, since it has a three-dimensional waveguide structure and operates in a single mode, it has the advantage that it can be used by directly connecting with other optical circuit elements. Furthermore, the waveguide type optical isolator of the present invention has the advantage of being able to sufficiently operate with a magnetic field as strong as several magnets compared to a normal bulk type optical isolator, so that the device can be made smaller and more economical. .

[Brief explanation of the drawing]

FIG. 1 shows the basic configuration of the waveguide type optical isolator of the present invention, in which (A) is a plan view, (B) is a side view, and (C) is a first
Figure 2 (A) and (B) are explanatory diagrams showing the operation of the wavelength multiplexing/demultiplexing section of the waveguide type optical isolator of the present invention, Figure 3 (A) is a view seen from the B side of Figure (A).
- (H) are explanatory diagrams showing the operation of the mode selection section of the waveguide type optical isolator of the present invention, and Fig. 4 shows the structure of the magnetic garnet-embedded waveguide 13 of the waveguide type optical isolator of the present invention. A) is a plan view, (B) is a view seen from the B side of FIG. 1 (A), (C) is a side view, and FIG. The structure is shown, (A) is a plan view, (B) is the first
FIG. 6, which is a view seen from the B side of FIG.
．． 7' structure, (A-1), (A-2) are plan views, (B-1), (B-2) are side views, (C-1),
(C-2) is a view seen from the B side of FIG. 1(A), FIG. 7 is an enlarged view showing the structure of the reciprocal mode converter 9 used in the embodiment of the present invention, and (A) is a planar view. Figure, (B) is Figure 1 (^)
FIG. 8 is an enlarged view showing the structure of the magnetic garnet-embedded waveguide 13 used in the embodiment of the present invention, (A) is a plan view, and (B) is a view from the B side of FIG. FIG. 9 is an explanatory view showing the structure of a waveguide type optical isolator according to an embodiment of the present invention, and FIG. 10 is a perspective view showing the structure of a bulk type optical isolator. 1.1'...Polarizer 2...Magneto-optic crystal 3...Light beam 4...Substrate 6...Normal to film surface 7.7'...Mode selection unit 8...Non Reciprocal mode converter 9...Reciprocal mode converter 10...Clad layer 11 Core portion 12...Stress release groove 13...Magnetic garnet embedded waveguide 14...
Guide block 15...Magnetic field 16 in a direction parallel to the light beam...Birefringent principal axes 18, 18', 18'', 18'''' of the core portion in the reciprocal mode converter 9...Light directionality for the mode selection section Coupler 19, 19'...Single mode waveguide section 21, 21' constituting the Matsuhatsu Ender interferometer...Wavelength multiplexing/demultiplexing section 22...Optical directional coupler 23 for wavelength multiplexing/demultiplexing section ...Buried layer 24...Lower cladding layer

Claims (1)

[Claims] 1. A silica-based glass single mode waveguide whose core portion is embedded in a cladding layer formed on a substrate, and an optical directional coupler formed at a light input end and a light output end of the silica-based waveguide. A Mach-Zehnder device comprising a wavelength multiplexing/demultiplexing section, two optical directional couplers formed in the silica single mode waveguide adjacent to the wavelength multiplexing/demultiplexing section, and two single mode waveguides. A mode selection section using an interferometer, a non-reciprocal mode converter consisting of a single mode waveguide made of magnetic garnet sandwiched between the mode selection section, and a part of the cladding layer having a predetermined length along the core section. What is claimed is: 1. A waveguide type optical isolator comprising: a reciprocal mode converter in which a stress release groove is formed;