JP2862630B2 - Waveguide type optical isolator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical isolator for preventing unnecessary reflected light from returning to a laser, an optical amplifier, or the like in the field of optical communication.

(Prior Art) An optical isolator is an important element used in the field of optical communication for the purpose of stabilizing a laser light source, suppressing line echo, preventing noise from occurring in an optical amplifier, and the like. Conventionally, as this type of element, as shown in FIG. 10, what is called a bulk type in which a magneto-optical crystal 2 and two polarizers 1, 1 'are combined has been put to practical use. In particular, optical isolators that do not depend on the polarization direction of signal light such as coherent communication include polarizers 1 and 2.
A polarization independent optical isolator using a wedge prism of birefringent crystal is used for 1 '. However, this type of optical isolator comprises a magneto-optical crystal 2 and a polarizer 1,
Since 1 'is expensive, the optical isolator itself is also expensive, and since it is coupled to a fiber or the like using a lens, the overall dimensions are, for example, 70 mm × 15 mm × in the case of a polarization independent optical isolator There were drawbacks such as being about 15 mm in size and making alignment difficult when combined with lenses and fibers.

Recently, in the field of optical communication, wavelength multiplexing communication has been proposed in which the wavelength of signal light transmitted in an optical fiber is multiplexed in order to increase the amount of information transmitted through one optical fiber. A communication system using an Er-doped fiber amplifier that amplifies the signal light intensity by propagating the signal light together with the pump light having different wavelengths in the Er-doped fiber to increase the repeaterless transmission distance has been proposed. . These new communication systems require wavelength multiplexed optical isolators that operate in two or more wavelength bands simultaneously. In particular, in a communication system using an Er-doped fiber amplifier, a polarization independent and wavelength multiplexing type optical isolator is required. However, in the optical isolator shown in FIG. 10, since the garnet crystal used for the magneto-optical crystal 2 has wavelength dependence, the element of the magneto-optical crystal 2 required for the element to operate for each wavelength is present. The length l 'is different. For this reason, in one optical isolator, for example, a signal wavelength of 0.8 μm
Band, 1.3 μm band, 1.5 μm band, etc., and the excitation light wavelength of 0.1 in the communication system using Er-doped fiber amplifier.
There is a disadvantage that sufficient operation cannot be simultaneously performed in two or more wavelength bands of the 98 μm band and the signal light wavelength band of 1.5 μm.

In order to realize a wavelength multiplexing type optical isolator, the tenth
A wavelength demultiplexing element is formed in front of the optical isolator shown in the figure, the incident light is separated for each wavelength, and the light is incident on a single-wavelength optical isolator set to operate optimally for each wavelength. The configuration may be such that the wavelength is multiplexed by the wavelength multiplexing element formed at the emission end of the single wavelength optical isolator. Such a wavelength multiplexing / demultiplexing device can be realized by combining an interference filter using a dielectric multilayer film or the like with a prism,
By using this, a wavelength division multiplexing type optical isolator can be easily realized. However, since such a wavelength division multiplexing type optical isolator is a bulk type in which individual components are combined, alignment is difficult when combining the individual components, and since it is coupled to an optical fiber using a lens, an optical For example, when a bulk-type optical isolator is used as a single-wavelength type isolator, there is a problem that the entire isolator becomes large, for example, about 110 mm × 25 mm × 15 mm. On the other hand, recently, it has been proposed to fabricate a waveguide-type wavelength multiplexing / demultiplexing device by forming an optical directional coupler in a silica-based waveguide (N. Takato et al. Electronics L.
etters, vol. 22, No. 6, p. 321). By using this waveguide-type wavelength multiplexing / demultiplexing device, it is possible to realize a waveguide-type wavelength-division multiplexing optical isolator. There is no.

(Problems to be Solved by the Invention) The present invention provides a small, low-cost, three-dimensional, polarization-independent, non-polarization operating element that can be used for practical use at a small number of wavelengths. An object of the present invention is to provide a waveguide type optical isolator.

(Means for Solving the Problems) FIG. 1 shows n signal light wavelengths λ ₁ , λ ₂ ,..., Λ _n of the present invention.
(A) is a plan view, (B) is a side view, and (C) is (A).
FIG. 2 is a diagram viewed from the side B of FIG.
1 'denotes a wavelength multiplexing / demultiplexing unit, 7 and 7' denote mode selection units, 8 denotes a non-reciprocal mode converter, and 9 denotes a reciprocal mode converter.

The wavelength multiplexing / demultiplexing units 21 and 21 'are configured by connecting n-1 wavelength multiplexing / demultiplexing unit optical directional couplers 22 in series as shown in FIGS. 2 (A) and 2 (B). Have been. The optical directional coupler 22 for the n-th wavelength multiplexing / demultiplexing unit in FIG. 2 adjusts the wavelength by adjusting the length of the coupling unit and the distance between the two waveguides constituting the optical directional coupler. It is set so that it functions as a 100% power transfer device only for the signal light of λ _{n + 1} and does not transfer any power for the signal light of other wavelengths. As a result, when the wavelength multiplexing / demultiplexing units 21 and 21 'are used as wavelength demultiplexing units, as shown in FIG. 2 (A), the wavelengths λ ₁ , λ ₂ , , Λ _n are demultiplexed into n waveguides in the order of λ ₁ , λ ₂ ,..., Λ _n from the top of FIG. 2 (A). Also when used as a wavelength multiplexing unit, as shown in FIG. 2 (B), lambda ₁ from the top of FIG. 2 (B), lambda _2, ..., propagated through the n-number of waveguides in the order of lambda _n The function of multiplexing the reflected signal light into one waveguide is shown.

The mode selectors 7, 7 '
Two optical directional couplers 18 and 18 'for the mode selector, respectively
Mach-Zehnder interferometers comprising n ″ and 18 ″, 18 and single-mode waveguide sections 19, 19 ′ are arranged in parallel in n pieces. These Mach-Zehnder interferometers are shown in FIG. In A), it is set so as to function at wavelengths λ ₁ , λ ₂ ,..., Λ _n in order from the top, ie, in FIG.
N th Mach-Zehnder interferometer of the optical directional coupler from the top 18, 18 ', 18 ", 18, by halving the coupling length the length of the coupling portion at the wavelength lambda _n, functions as a 3dB coupler In addition, the single-mode waveguide sections 19 and 1 of the n-th Mach-Zehnder interferometer from the top are set as follows.
The two waveguides 9 ′ are such that the relative phase difference of the guided light propagating at each wavelength λ _n is π for the quasi-TE mode in which the main component of the electric field is perpendicular to the normal 6 to the film surface. And the main component of the electric field is set to be 0 in the quasi-TM mode parallel to the normal 6 of the film surface. As a result, in FIG. 1 (A), the n-th Mach-Zehnder interferometer from the top operates at the wavelength λ _n as shown in FIGS. 3 (A) to 3 (H), and has 100% power for the TM mode. functions as a shifter as a whole mode selector 7, 7 'is the wavelength lambda _1, lambda _2, ..., serving as a TE-TM mode demultiplexers and TE-TM mode multiplexer for the signal light of lambda _n .

The non-reciprocal mode converter 8 is constituted by a magnetic garnet embedded waveguide 13 having 2n cores arranged in parallel, to which a magnetic field 15 in a direction parallel to the light beam 3 is applied by a coil or a magnet. FIG. 4 is a diagram showing the configuration of the magnetic garnet embedded waveguide 13. C and D in FIG. 4 correspond to C and D in FIG. 1 (A). As shown in FIG. 4 (A), the 2n core portions 11 are divided into n sets of 2 adjacent to each other. In FIG. 4, the n-th set of core portions from the left has a Faraday rotation coefficient at a wavelength λ _n of a magnetic garnet thin film constituting the magnetic garnet embedded waveguide 13 of θ _F (λ _n ) (deg / cm). Then the waveguide length l _n
Is designed to satisfy the relationship of | θ _F (λ _n ) | · l _n = 45 °, and is set so as to perform a TE-TM nonreciprocal 50% mode conversion operation at the wavelength λ _n . Therefore, the magnetic garnet embedded waveguide 13 has wavelengths λ ₁ and λ
₂ ,..., Operates as a TE-TM nonreciprocal 50% mode converter corresponding to _n . In addition, magnetic garnet embedded waveguide 13
The respective core portion is set to single-mode propagation in the corresponding wavelength lambda _n, and so maintain good linear polarization resistance, so that the propagation constants of the quasi-TE mode and the quasi-TM mode matches Is set to 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, resulting in a decrease in element characteristics.

The magnetic garnet embedded waveguide 13 is fitted between the mode selector 7 and the reciprocal mode converter 9 using the guide block 14 with the substrate 4 facing upward.

5A and 5B show the configuration of the reciprocal mode converter 9, wherein FIG. 5A is a plan view, FIG. 5B is a diagram viewed from the side B in FIG. 1A, and C and C in FIG. D corresponds to C and D in FIG. As shown in FIG. 5, the reciprocal mode converter 9 is provided on the cladding layer 10 near the 2n cores 11 arranged in parallel. The direction of the main shaft 16 is perpendicular to the light propagation direction, and is inclined 22.5 degrees toward the stress release groove 12 from the direction of the normal 6 of the film surface of the substrate,
By setting the lengths of the stress release grooves 12 to predetermined lengths, 2n waveguide-type half-wave plates are formed, each of which functions as a TE-TM reciprocal 50% mode converter. Is set. At this time, the 2n waveguide-type half-wave plates are two adjacent ones from the left in FIG. 5 (A).
Λ ₁ , λ ₂ ,...,
to optimally operate at lambda _n, the length and width of the stress relief groove 12, the distance between the core portion 11 and the stress relief groove 12 is set.

The operation principle of the waveguide type optical isolator of the present invention will be described with reference to FIG. For convenience of explanation, in FIG. 1A, of the 2n waveguides arranged in parallel with the mode selectors 7 and 7 ', the non-reciprocal mode converter 8, and the reciprocal mode converter 9, the end of the waveguide Are coupled to the wavelength multiplexing / demultiplexing units 21 and 21 ', and those not coupled are referred to as waveguides b. Also for convenience, when the sign of theta _F of the magnetic garnet will be described a positive (theta _F code in the same direction in parallel the direction of the traveling direction and the external magnetic field 15 of the light constituting the magnetic garnet embedded waveguide 13 The direction in which the electric field vector of light rotates clockwise as viewed from the light source side is defined as positive). Λ ₁ , λ ₂ ,..., Λ incident on the core portion from A in FIG.
_The waveguide light in which n wavelengths of n are multiplexed is divided into n waveguides by wavelength in the order of λ ₁ , λ ₂ ,..., λ _n in FIG. It is split. The quasi-TE mode is applied to the waveguide a by the mode selector 7 and the quasi-TM
The mode is separated by the waveguide b and enters the non-reciprocal mode converter 8. In each waveguide, the non-reciprocal mode converter 8 converts the guided light into 50% mode, and 50% quasi-TE mode.
The light is converted into quasi-TM mode 50% guided light and propagated to the reciprocal mode converter 9. In the reciprocal mode converter 9, the guided light becomes quasi-TE mode 100% in the waveguide a and the quasi-TM mode 1 in the waveguide b.
After being converted to 00%, the light enters the mode selection unit 7 '. In the mode selection unit 7 ', the quasi-TM mode of the waveguide b shifts 100% in power to the waveguide a, and exits to the wavelength multiplexing / demultiplexing unit 21' together with the quasi-TE mode. N incident on the wavelength multiplexing / demultiplexing part 21 '
The guided lights having different wavelengths traveling through the two waveguides are multiplexed into one waveguide and output to B in FIG. 1 (A). Lambda ₁ reverse to the incident on the core portion 11 from B, lambda _2, ..., lambda guided light of n wavelengths are multiplexed for _n, the first view in the wavelength division unit 21 '(A)
From the top, λ ₁ , λ ₂ ,..., Λ _n are split into n waveguides for each wavelength in order from the top. The guided light separated for each wavelength is separated into the quasi-TE mode into the waveguide a and the quasi-TM mode into the waveguide b by the mode selector 7 ′, and enters the reciprocal mode converter 9. In the reciprocal mode converter 9, the guided light passes through 50
% Mode conversion, and converted into guided light of quasi-TE mode 50% and quasi-TM mode 50%.

Next, the guided light is converted to a quasi-TM mode 100% in the waveguide a in the nonreciprocal mode converter 8 and a quasi-TE mode 100% in the waveguide b.
%, And enters the mode selection unit 7. The quasi-TM mode of the waveguide a is set to 10
Since the power shifts to 0%, the quasi-TE mode originally guided in the waveguide b cannot be emitted to the wavelength multiplexing / demultiplexing unit 21, and the guided light emitted to A in FIG. 0, and the optical circuit shown in FIG. 1 (A) has wavelengths λ ₁ , λ ₂ ,.
_It functions as a polarization independent optical isolator operating at n wavelengths of n.

Examples Hereinafter, as examples of the present invention, wavelengths of 1.3 μm and 1.55 μm will be described.
A waveguide type optical isolator that operates simultaneously on m guided lights will be described.

6 (A-1), (B-1), (C-1) and (A-2), (B-2), (C-2) show wavelength combining used in the embodiment of the present invention. 7A and 7B are enlarged views of the wave sections 21 and 21 'and the mode selection sections 7 and 7'.
FIG. In this embodiment, a silicon substrate having a thickness of 0.7 mm is used for the substrate 4 in these portions, and quartz glass is used for the core portion 11 and the cladding layer 10. The thickness of the clad layer 10 was 50 μm, and the center of the core 11 was 25 μm from the substrate surface. When connecting the magnetic garnet-embedded waveguide 13, the core 11 is formed in the same square shape as the core of the magnetic garnet-embedded waveguide 13 so as to reduce the connection loss. Also for single mode propagation,
The relative refractive index difference between the core 11 and the cladding layer 10 was set to 0.25%.
In the wavelength multiplexing / demultiplexing units 21 and 21 ', the length and distance of the coupling portion of the two waveguides of the optical directional coupler 22 are adjusted so as to function as a wavelength division multiplexer of 1.3 μm and 1.55 μm. Thus, the setting is made such that the power shifts to 100% for the guided light having a wavelength of 1.3 μm and does not shift at all for the guided light having a wavelength of 1.55 μm. In FIG. 6 (A-1), the mode selectors 7 and 7 'use the waveguide a so that the Mach-Zehnder interferometer composed of the waveguide causes a 100% power transition of the quasi-TM mode at a wavelength of 1.55 μm. The propagation constants of the quasi-TE mode and quasi-TM mode of the waveguide b and the quasi-TM mode were made equal at a wavelength of 1.55 μm. Similarly, in FIG. 6 (A-1), a Mach-Zehnder interferometer composed of a waveguide has a wavelength of 1.
At 3 μm, 100% power transition of quasi-TM mode occurs.
The propagation constants of the waveguide and the quasi-TE mode and the quasi-TM mode of the waveguide were made equal at a wavelength of 1.3 μm. In the reciprocal mode converter 9, in order to set the angle at which the principal axis of the birefringence of the core 11 is inclined from the substrate surface normal 6 to 22.5 degrees, FIG.
The distance S from the center of the core part 11 to the stress release groove 12 shown in
It was 35 μm. The width T of the stress release groove 15 was 200 μm. During Figure 7 (A), waveguides, and, for the length L ₁ and L ₂ of the stress relief groove 12 to have a function as a 50% mode converter with a wavelength of 1.55μm and the wavelength 1.3μm against, 3.1 mm and 2.6 mm. The structure of such an optical waveguide and a stress release groove is based on a flame etching reaction of a glass forming material gas such as SiCl ₄ , TiCl _4, etc., and a dry etching process typified by reactive ion etching (RIE). Can be formed by a well-known processing method in combination with

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 viewed from the B side in FIG. 1 (A). FIG.
23 denotes a buried layer, and 24 denotes a lower cladding layer. In this embodiment, in order to make the propagation constants of the quasi-TE mode and the quasi-TM mode equal, the shape of the core part 11 is rectangular, and the buried layer 23 and the lower cladding layer 24 have the same refractive index. The width W of the core portion 11 was 8 μm. In order to manufacture this magnetic garnet waveguide, a two-layer garnet thin film is formed on the substrate 4 by a liquid phase epitaxy method (LPE method), a sputtering method, or the like, and the core portion 11 is subjected to ion milling and reactive ion etching. It can be manufactured by patterning by an etching method such as (RIE) and finally depositing a buried layer 23 by an LPE method, a sputtering method or the like.

As a specific configuration of the magnetic garnet embedded waveguide 13, a 0.5 mm-thick GGG (gadolin gallium garnet) substrate is used as the substrate 4, and a substitution type YIG ( Yttrium iron garnet) was used. The specific composition of substitutional YIG is
In order to reduce the external magnetic field 15 necessary for the device to operate, it is desirable that the composition be in-plane magnetization, so (Sc, Ga) YIG
And (La, Ga) YIG were used. The refractive indices of the core portion 11, the buried layer 23, and the lower cladding layer 24 are changed by changing the Ga substitution amounts so that the core portion 11 propagates in a single mode and the width W is 8 μm. Set to. In FIG. 7A, the waveguide length of the magnetic garnet embedded waveguide 13 and the element lengths l ₁ and l
_2, and these substituted YIG of theta _F wavelength 1.55 .mu.m 1.
Since it is 160 deg / cm and 206 deg / cm at 3 μm, about 50% mode converter operates at each wavelength.
2.8 mm and 2.2 mm.

The magnetic garnet-embedded waveguide 13 thus obtained is combined with the wavelength multiplexing / demultiplexing units 21 and 21 'and the mode selecting units 7, 7' shown in FIGS. 6 (A-1) and 6 (A-2). In addition, the single mode optical waveguide having the reciprocal mode converter shown in FIG. 7 is placed along the guide block 14 shown in FIG. 1A with the buried layer 23 shown in FIG. After mounting, positioning, and fixing with an adhesive, a waveguide-type optical isolator shown in FIG. 9 was formed.

The external magnetic field 15 is the magnetic garnet embedded waveguide 13.
Substituted YI was used to ^- the sign of theta _F of G is positive, 9
The direction was applied from the direction shown in the figure. The overall size of the waveguide type optical isolator of this embodiment is about 75.9 mm in element length (the wavelength multiplexing / demultiplexing parts 21 and 21 ′ are 20 mm × 2, and the mode selecting parts 7 and 7 ′ are about
15.0mm x 2, non-reciprocal mode converter 8 is about 2.8mm, reciprocal mode converter 9 is 3.1mm) x 5.0mm x 1.25mm thick,
It is smaller than the case where a wavelength multiplexing type optical isolator is constituted by a bulk type.

The isolation ratio of the waveguide type optical isolator of this example was measured by actually connecting to an optical fiber. The waveguide type optical isolator of this embodiment has an isolation ratio that saturates at a magnetic field number of 10 Oe, has a saturation value of 21 dB and 20 dB at wavelengths of 1.55 μm and 1.3 μm, and has a high isolation ratio at both wavelengths. And operates as a two-wavelength multiplexing type optical isolator. 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.

By the way, in the above embodiment, the same substitutional YIG as the lower cladding layer 24 was used as the material of the burying layer 23 of the magnetic garnet buried waveguide 13, but the refractive index was made equal to the refractive index of the lower cladding layer 24. The same effect was obtained by using an amorphous material such as Ta ₂ O ₅ or Nb ₂ O ₅ . In the above embodiment, 1.55 μm and 1.3 μm are used as the guided light wavelengths. However, the present invention can be applied to other wavelengths of guided light to realize a wavelength multiplexing type optical isolator having similar performance. Have confirmed.

(Effects of the Invention) As described above, the waveguide type optical isolator of the present invention can operate simultaneously on a plurality of wavelengths of guided light unlike the waveguide type optical isolators proposed so far. 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, a polarization-independent and wavelength-multiplexed isolator is required.
There is an advantage that it is suitable for use in an optical communication system using an Er-doped fiber amplifier. Further, since it has a three-dimensional waveguide structure and operates in a single mode, there is an advantage that it can be used by directly connecting to another optical circuit element. Further, the waveguide type optical isolator of the present invention operates sufficiently with a magnetic field having an intensity several orders of magnitude smaller than that of a normal bulk type optical isolator, and thus has an advantage that the element can be reduced in size and economical. .

[Brief description of the drawings]

1A and 1B show a basic configuration of a waveguide type optical isolator of the present invention, wherein FIG. 1A is a plan view, FIG. 1B is a side view, and FIG.
2 (A) and 2 (B) are explanatory views showing the operation of the wavelength multiplexing / demultiplexing unit of the waveguide type optical isolator of the present invention, FIG. 3 (A). 4 (H) are explanatory diagrams showing the operation of the mode selection section of the waveguide type optical isolator of the present invention. FIG. 4 shows the structure of the magnetic garnet embedded type waveguide 13 of the waveguide type optical isolator of the present invention. 1A is a plan view, FIG. 1B is a view from the side B in FIG. 1A, FIG. 1C is a side view, and FIG. 5 is a view of the reciprocal mode converter 9 of the waveguide type optical isolator of the present invention. 2A shows a structure, FIG. 2A is a plan view, and FIG.
FIG. 6A is a view from the B side of FIG. 6A, and FIG. 6 is a wavelength multiplexing / demultiplexing unit 21, 21 ′ used in the embodiment of the present invention.
And the structure of the mode selectors 7 and 7 ', (A-1),
(A-2) is a plan view, (B-1) and (B-2) are side views, (C-1) and (C-2) are views seen from the B side in 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, where (A) is a plan view, (B) is a view seen from the B side in FIG. 1 (A), FIG. 8 is an enlarged view showing the structure of the magnetic garnet embedded waveguide 13 used in the embodiment of the present invention, wherein FIG.
(B) is a view from the B side of FIG. 1 (A), FIG. 9 is an explanatory view showing a configuration of a waveguide type optical isolator according to an embodiment of the present invention, and FIG. 10 is a configuration of a bulk type optical isolator. FIG. 1,1 '... Polarizer 2 ... Magneto-optical crystal 3 ... Light beam 4 ... Substrate 6 ... Normal of film surface 7,7' ... Mode selector 8 ... Non-reciprocal mode converter 9 ... Reciprocal mode conversion unit 10 Cladding layer 11 Core 12 Stress release groove 13 Magnetic garnet embedded waveguide 14 Guide block 15 Magnetic field parallel to light beam 16 Reciprocal mode conversion Birefringent principal axes 18, 18 ', 18 ", 18 of the core portion in the section 9 ... Optical directional coupler for mode selection section 19, 19' ... Single mode waveguide section constituting Mach-Zehnder interferometer 21, 21 ': wavelength multiplexing / demultiplexing unit 22: optical directional coupler for wavelength multiplexing / demultiplexing unit 23: buried layer 24: lower cladding layer

Claims (1)

(57) [Claims] 1. A quartz glass single-mode waveguide having a core buried in a cladding layer formed on a substrate, and a light directivity formed at a light incident end and a light emitting end of the quartz waveguide. A wavelength multiplexing / demultiplexing section comprising a coupler, and two optical directional couplers and two single mode waveguides formed in the quartz-based single mode waveguide adjacent to the wavelength multiplexing / demultiplexing section. A mode selector using a Mach-Zehnder interferometer, a non-reciprocal mode converter consisting of a single mode waveguide of a magnetic garnet sandwiched between the mode selectors, and a part of the cladding layer along the core. A waveguide type optical isolator comprising: a reciprocal mode converter having a stress release groove having a predetermined length.